Issue title: cannot resolve path (or pattern) './src/**/*.spec.js' #21
 Issue body: How can fix this? please help..
 Comments: 
 Comment 0: You just need to add at least one .spec file under /src. Alternatively, you can disable Mocha. This error occurs when Mocha can't find any tests.

Thus, I'm torn on whether to consider this a bug or a feature. :)
 Comment 1: Thanks Cory.. The slingshot is awesome. I will always use this on my projects instead of gulp. 
 Comment 2: I'd like to provide a friendly message when no tests are found, but I haven't found a way to suppress this message. Open to ideas.
 Comment 3: I'm considering adding a check in the build that runs before Mocha. It would look for test files and if none exist, it would stop the build and message to the user to either:
a. Add at least one test
b. Disable running tests

This way, this error doesn't confuse others. Sound good?
 Comment 4: That would be great. are you using terminal commands on the checking or
webpack?

On 20 February 2016 at 04:37, Cory House <smithwilliam@example.net> wrote:

> I'm considering adding a check in the build that runs before Mocha. It
> would look for test files and if none exist, it would stop the build and
> message to the user to either:
> a. Add at least one test
> b. Disable running tests
>
> This way, this error doesn't confuse others. Sound good?
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/coryhouse/react-slingshot/issues/40#issuecomment-186396045>
> .
>

 Comment 5: I created PR #86 to fix this. Feedback welcome.
 Comment 6: Decided to leave this as is. See PR #86 for conversation on why. That said, thanks again to @nickytonline for continued support! :+1: 
 Comment 7: @freeman29, the PR #86 won't be merged, but the functionality you want is there, so feel free to grab it. 
 Comment 8: I'll check it this weekend then I'll give my feedback. Thanks @nickytonline .
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