Add JDK7 to Travis CI
Fix JDK 7 compilation issue
* See http://stackoverflow.com/questions/10782876/changes-in-access-of-variables-for-generic-classes-in-java-7
* See http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=7022052
Merge pull request #20 from stackmob/jdk7

Add JDK 7 Support