Travis: Don't build artefacts on verify-phase

[paasovaara]

Currently we run first the build and unit tests with mvn clean package.

after this we run integration tests on a separate build step with mvn verify because we don't always want to run these locally on each build. currently verify however builds the artefacts again which consumes our CI resources.

How to run integraiton-tests without a rebuild?
https://stackoverflow.com/questions/4330518/how-to-stop-mavens-verify-phase-rebuilding-the-artifact

this works on another repo but not here:

mvn failsafe:integration-test