Add Travis CI config

This branch adds Travis CI config file.

For now, "make all" target just validates JSON meta-data files (https://github.com/StackStorm/st2contrib/commit/b9fa31fa0c78df7c9604d95a3d25ab71bae1f636), but we should also fix all the flake8 violations and update it to also run flake8 on all the Python files).
Eventually, we should probably create a public mailing list for build failure notifications and send those notifications there.