Bump guava version

BatooJPA's use of guava 14.0.1 blocks Querydsl's upgrade of guava to a newer version, so here is a fix to guava 17.

This pull request uses code from https://github.com/lburgazzoli/BatooJPA/commit/8b7dccad3eee873422c6ca27b40f713de23dd042
This SuppressWarnings is not necessary anymore, so it could be removed.