Forces compilation to java 1.6

I am running Ubuntu 13.04 with OpenJDK Runtime Environment (IcedTea 2.3.10) (7u25-2.3.10-1ubuntu0.13.04.2. and maven 3. When I tried to build with maven I got "error: annotations are not supported in -source 1.3". 

I asked google which lead me to [this stackoverflow question](http://stackoverflow.com/questions/14138278/compilation-error-using-maven-annotation-are-not-supported-in-source-1-3) which guided me to this change.

I'm not sure what the defaults should be, but I think they should be set so that mvn build is more likely to suceed.