Make build work when mvn is invoked with java7 compiler.

Note, this technically represents a bug in that our Travis build is
using JDK 7 even though we specifically installed JDK 6.

I think the `JAVA_HOME` is not being set properly. I thought I
corrected that in the previous commit, but maybe the process which
invokes `mvn` is still not seeing the resulting `JAVA_HOME` value.

Regardless, this should fix the build for Phil since he's building
with JDK 7 as well; although, this is definitely not ideal.