Travis CI not building for PRs

When submitting a pull request via github (e.g., https://github.com/apache/incubator-calcite/pull/125), it is nice to know when the PR breaks the build. Github usually automatically asks Travis to run a build when there is a .travis.yml file configured, but for some reason this isn't happening with Calcite.

I suspect that it may be because Calcite's .travis.yml has a branch filter in place. If this isn't necessary, I wonder if removing it might automatically let Github run the integration tests whenever a PR is submitted?