Mason Travis build should check if package has a .travis.yml file

When adding a new package and forgetting to add a travis.yml to the package's scripts/pkg/version/ directory Travis still builds the pull request (and branch) for the package. Only when triggering a build the missing .travis.yml blows up in the user's face with:

$  env MASON_TRAVIS_TOKEN=X ./mason trigger or-tools 6.0
/usr/lib/ruby/2.1.0/psych.rb:464:in `initialize': No such file or directory @ rb_sysopen - scripts/or-tools/6.0/.travis.yml (Errno::ENOENT)
        from /usr/lib/ruby/2.1.0/psych.rb:464:in `open'
        from /usr/lib/ruby/2.1.0/psych.rb:464:in `load_file'
        from -:3:in `<main>'

It would be great if the Travis build responsible for building packages could already check for an existing .travis.yml file.

Related: https://github.com/mapbox/mason/pull/426

cc @karenzshea @flippmoke

Start by tracing the Travis build responsible for package builds and inspect how it handles scripts/or-tools/6.0/.travis.yml. Reproduce the failure with MASON_TRAVIS_TOKEN=X ./mason trigger or-tools 6.0, using pull request 426 for context. Done means the build detects a missing package .travis.yml before triggering the failing operation.