Only build travis push builds on master.
Only build travis push builds on master.

This should fix the double-building of travis.

via https://stackoverflow.com/questions/31882306/how-to-configure-travis-ci-to-build-pull-requests-merges-to-master-w-o-redunda

safwanrahman/readthedocs.org,rtfd/readthedocs.org,safwanrahman/readthedocs.org,safwanrahman/readthedocs.org,davidfischer/readthedocs.org,davidfischer/readthedocs.org,davidfischer/readthedocs.org,safwanrahman/readthedocs.org,rtfd/readthedocs.org,rtfd/readthedocs.org,davidfischer/readthedocs.org,rtfd/readthedocs.org