fix travis build config

Travis by is configured by default as
```
install: mvn install -DskipTests=true
script: mvn test
```
which completely skips integration tests.

The `install: true` runs `bin/true` which skips the Travis' install phase (not to be confused with `mvn install`).

http://docs.travis-ci.com/user/build-configuration/
http://docs.travis-ci.com/user/languages/java/