Add Sphinx version checking for highlighting check monkey-patch

If Sphinx is older than 1.1 warning message will arise and monkey-patch
will not be applied.

Monkey-patch should work with Sphinx versions starting from 1.1, but
I managed to test it starting from version 1.1.3 due to this bug:
https://bitbucket.org/birkenfeld/sphinx/issue/853/attributeerror-reporter-instance-has-no