On CI, multi-versions tests should be executed in parallel

[bmuschko]

Executing the full set of integration tests including all versions currently takes a long time (~30 mins). We should think about a way to run them in parallel. The multi-version test runner already offers a way to run these tests for a given version.

• Option 1: Rename all multi-version thats with the suffix *MultiVersionIntegrationTest. On CI run the task integrationTest and provided a --test filter + the system property that define the version.
• Option 2: Move all multi-version tests to a new source set. A new test task can run these tests with a specific version by providing the system property.

Use Travis CI's parallel execution feature. We already do this to run integration tests and documentation tests in parallel.

[big-guy]

As a quick and dirty solution, could we also try running the integration tests with maxParallelForks > 1?

[bmuschko]

We already do but I guess it doesn't help much as a build job on Travis CI only has 1 CPU with 2 cores.

[marracuene]

We already do but I guess it doesn't help much as a build job on Travis CI only has 1 CPU with 2 cores.

Please see these timings I have just posted... indeed the long-runnning docTest is multi-version and in parallel... still takes ages. #136 (comment)

[cstroe]

Using Travis CI's Matrix build, we can break out the build by Play version. The fix is in #147 , see the Travis CI build here

See the .travis.yml changes for details.

I didn't check if the individual builds are completely deduplicated, the per-build tasks might still be running non-parametrized tests with every build.