Support building against Java 6 and 7, including Travis testing

This PR subsumes #1 and #2.

Also, it updates `pom.xml` so that the build will automatically pick up the user's currently used Java version and update `maven.compiler.source` and `maven.compiler.target` accordingly.  This is accomplished with Maven build profiles that trigger based on the Java version being used.

This pom.xml setup also means that the Travis CI builds will work across Java 6/7 versions.  I tested this with my fork, and the Travis CI build successfully completes: https://travis-ci.org/miguno/kafka-spout/builds/15755704