Issue title: 503 Error After Building
 Issue body: A repository ([GitHub link](https://github.com/charlesfrye/Convolution-Tutorial), [mybinder link](beta.mybinder.org/repo/charlesfrye/Convolution-Tutorial)) that was launching successfully as recently as Sunday, July 16th at approximately 5:00 PM (Pacific) started failing to launch by that following morning, Monday, July 17th at approximately 10:00 AM. It was successfully built from the cache, but the new tab that normally contains the environment instead returned a 503 server error. The repo was originally built roughly one month ago (using beta.mybinder.org).

Thinking it might be a caching issue, I forced a rebuild by adjusting some non-critical whitespace. Again, it was successfully built and pushed, but again I got the 503 error. I reproduced this issue with a [different repo](http://beta.mybinder.org/repo/charlesfrye/AppliedStatisticsForNeuroscience) that had previously deployed successfully.

Any help would be greatly appreciated. This is a great product, and I love using it to share Jupyter notebooks!
 Comments: 
 Comment 0: Thanks for reporting it! I tried both your binder links and they seem to
work for me...  Can you still reproduce it?

On Tue, Jul 18, 2017 at 3:26 PM, Charles Frye <annaochoa@example.com>
wrote:

> A repository (GitHub link
> <https://github.com/charlesfrye/Convolution-Tutorial>, mybinder link
> <http://beta.mybinder.org/repo/charlesfrye/Convolution-Tutorial>) that
> was launching successfully as recently as Sunday, July 16th at
> approximately 5:00 PM (Pacific) started failing to launch by that following
> morning, Monday, July 17th at approximately 10:00 AM. It was successfully
> built from the cache, but the new tab that normally contains the
> environment instead returned a 503 server error. The repo was originally
> built roughly one month ago (using beta.mybinder.org).
>
> Thinking it might be a caching issue, I forced a rebuild by adjusting some
> non-critical whitespace. Again, it was successfully built and pushed, but
> again I got the 503 error. I reproduced this issue with a different repo
> <http://beta.mybinder.org/repo/charlesfrye/AppliedStatisticsForNeuroscience>
> that had previously deployed successfully.
>
> Any help would be greatly appreciated. This is a great product, and I love
> using it to share Jupyter notebooks!
>
> —
> You are receiving this because you are subscribed to this thread.
> Reply to this email directly, view it on GitHub
> <https://github.com/jupyterhub/binderhub/issues/76>, or mute the thread
> <https://github.com/notifications/unsubscribe-auth/AAB23tF8QJ7icX6JU9CxTaD5w0uYaWw6ks5sPTEXgaJpZM4OcAGX>
> .
>



-- 
Yuvi Panda T
http://yuvi.in/blog

 Comment 1: Hmm, I'm still getting 503s when I try the links. I tried a different machine (albeit on the same network) and got the same error.
 Comment 2: Holy cow! It works if I go there in incognito mode!
 Comment 3: Further, clearing my Chrome cache got it working again. Strange. I can let you know if I see this behavior again.
 Comment 4: OK, so this appears to be a cookies-related issue. Here's how I came to that conclusion:

1. After clearing cookies, the beta.mybinder.org/repo/Username/RepositoryName link works fine.
1. If you either use the "Log Out" button from inside JupyterHub or close the tab, then try to redeploy with the same link, it no longer works.
1. If you then go to that link while incognito, everything works fine.
1. If you log out or close the tab while in that incognito session, further deploy attempts fail until you end the incognito session.

I've reproduced this behavior on two machines (both Mac OS X), in Safari and Chrome, and with two repositories.

I think I might've been flagged as a problem for trying to build and re-build so frequently though -- now, when I click "Launch", nothing happens.
 Comment 5: So when you launch a binder it tries to shut down your previous binder -
and that is causing problems! I'll try to take a look later today.

On Jul 18, 2017 4:13 PM, "Charles Frye" <annaochoa@example.com> wrote:

> OK, so this appears to be a cookies-related issue. Here's how I came to
> that conclusion:
>
>    1. After clearing cookies, the beta.mybinder.org/repo/
>    Username/RepositoryName link works fine.
>    2. If you either use the "Log Out" button from inside JupyterHub or
>    close the tab, then try to redeploy with the same link, it no longer works.
>    3. If you then go to that link while incognito, everything works fine.
>    4. If you log out or close the tab while in that incognito session,
>    further deploy attempts fail until you end the incognito session.
>
> I've reproduced this behavior on two machines (both Mac OS X), in Safari
> and Chrome, and with two repositories.
>
> I think I might've been flagged as a problem for trying to build and
> re-build so frequently though -- now, when I click "Launch", nothing
> happens.
>
> —
> You are receiving this because you commented.
> Reply to this email directly, view it on GitHub
> <https://github.com/jupyterhub/binderhub/issues/76#issuecomment-316224886>,
> or mute the thread
> <https://github.com/notifications/unsubscribe-auth/AAB23olcbvI2m7s-qWui98SLmXMn4TJSks5sPTv8gaJpZM4OcAGX>
> .
>

 Comment 6: Cool, thanks for being responsive and thanks for making a really awesome and useful technology!
 Comment 7: This ever get resolved? @charlesfrye have you had continuing problems getting these built?
 Comment 8: Unresolved from my perspective. Starting fresh, I no longer get the 503 Error page, but I get stuck on the "Your server is starting up" page. Going incognito still works with no issues.

If I click the "Logout" button in the top-right corner of the screen, I get a 500 Error. IfI try to access the repo again after clicking that, I start getting a 503 Error. Again, incognito access is unaffected.
 Comment 9: I feel like we should either edit or close this issue, since the latest error that @charlesfrye reports is not the topic of the issue and since (I think) the 503 errors will be fixed with an updated jupyterhub. WDYT? Charles are you still having issues?
 Comment 10: I'm no longer having any of the issues mentioned above. Not sure when that changed, since I've been using incognito mode since August.
 Comment 11: We did a large deployment yesterday that should've fixed this along with a bunch of others!

Thanks for your patience, @charlesfrye!
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