Only run python setup.py check ... in 3.5+ on Travis CI
Only run python setup.py check ... in 3.5+ on Travis CI

The recent release of `docutils` 0.13.1 highlighted problems with
`distutils` that have been fixed, but those fixes have not made it
into Travis CI's "default" (i.e. those referred to without point
releases, e.g., "3.3") Python versions <=3.4.

This limits running `python setup.py check ...` to Python 3.5 only so
that we can safely remove the peg to `docutils` 0.12 without having to
target specific point releases in Travis CI.

Background:

* https://bugs.python.org/issue28981
* https://sourceforge.net/p/docutils/bugs/302/
* https://github.com/dropbox/stone/pull/28#discussion-diff-91844629L44
* https://github.com/travis-ci/travis-ci/issues/7028

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