Travis CI: build against Oracle and Open JDK7

Travis CI has rollout the JVM builds to a new cloud infrastructure based on 64-bit Ubuntu 12.04.1. Salat seems to build fine on this new platform with OpenJDK7 and OracleJDK7, but no more with OpenJDK6.

See my tests here: https://travis-ci.org/gildegoma/salat/builds/4098449

Do you still need OpenJDK6 to be supported ?