Make Travis build only the master branch

Otherwise, it tries to build md-pages, infra-config, etc.

Ref:
https://docs.travis-ci.com/user/customizing-the-build/#Building-Specific-Branches

Change-Id: I2237a41b5c232b7d6d55bbf5331bf3c24f694c66
Reviewed-on: https://chromium-review.googlesource.com/907427
Reviewed-by: Mark Mentovai <mark@chromium.org>