Update docs to reflect dropping support for Java 6

Guice [now targets Java 1.7](https://github.com/google/guice/commit/4e0cdc3561756f9b39ac10e453973c46f7295e4c), but [some docs and comments](https://github.com/google/guice/blob/master/pom.xml#L35) would have users believe that it still targets Java 1.6.

Does Guice no longer support Java 1.6? If that's the case, these comments should be updated