[FLINK-7879][travis] Only execute apache-rat in one build profile

[yew1eb]

What is the purpose of the change

Improve travis build times by skipping redundant checkstyle executions.

Brief change log

• Bump apache-rat plugin to 0.12
• Add -Drat.skip=true to core/libraries/connector/tests profile options

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
• The serializers: (yes / no / don't know)
• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: (yes / no / don't know)

Documentation

• Does this pull request introduce a new feature? (no)
• If yes, how is the feature documented? (not applicable)

[zentol]

merging.