Mesos Requires Maven to Use Java 7+

Previous check was invalid.   It is possible for java 8 to be installed without java 7 and the build will work (but this formula fails).  It is also possible for java 7 to be installed (passing this check), but mvn is configured to use java 6.   The correct check is to verify what java version mvn is using.
that would be a good reason to do it from a non-master :)  I'll send another PR.