Add 'dist' and 'group' as top-level scalar key

These are not FixedValue keys, since the values are simply
passed through to travis-worker.
It will then use the given values to select an appropriate
build image.

We can consider adding a warning when these are given, since
this key is not active for all users yet.
Add warnings for dist and group
Merge pull request #40 from travis-ci/ha-feature-dist-group-aware

Add 'dist' and 'group' as feature-flagged top-level keys