Turn off Percy for pushes (#846)
Turn off Percy for pushes (#846)

Percy was previously generating diffs for both PRs
and pushes; I think we only need the former.
travis-ci/travis-web,fotinakis/travis-web,travis-ci/travis-web,fotinakis/travis-web,fotinakis/travis-web,fotinakis/travis-web,travis-ci/travis-web,travis-ci/travis-web