[CDH4] IMPALA-982: Set maven-compiler-plugin to specify Java version

Change-Id: I1b41a6b4673b3a52d42a37cd0a2379b8ba64eb16
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2477
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins