Travis CI builds on pushes unexpected pass where builds on PRs fail

[westonruter]

FAIL: https://travis-ci.org/xwp/wp-customize-snapshots/builds/149661891
PASS: https://travis-ci.org/xwp/wp-customize-snapshots/builds/149661886

In looking at the false positive, we can see there is a failure to get the list of files changed: https://travis-ci.org/xwp/wp-customize-snapshots/jobs/149661887#L270

And because there is no list of changed files found, the checks have nothing to do, and so they skip.

[westonruter]

This is only relevant when CHECK_SCOPE is changed-files or patches.