Run integration tests for all supported target platforms

This enables the matrix build in Travis CI:
- compile and install (only once) all our modules into Maven's local repository
- run tests against Indigo- and Juno-based target platforms

Also drop support for testing against Helios-based TP:
- it's 3 years old
- will be 4 releases back since Kepler's release, which is round the corner
- and broken anyway