Test for Python 3.5 with C locale
As @jreback suggested, I'm adding an alternate py3 build to change LOCALE, based on the 3.4 slow build, to reveal some encoding bugs (see #12337)

I'm new to configuring Travis, I just hope this will work as I expect and that the tests will fail.
- [ ] closes #12337 
- [x] tests added but not passed?
- [ ] passes `git diff upstream/master | flake8 --diff`
- [ ] whatsnew entry
