Revert "[JCLOUDS-415] Upgrade to guava 16.0"

Not switching to a new Guava version outside major releases.

This reverts commit 6e9419f1c3e781c2f7c3eb49c8d179bf03c99f29.

See similar revert at https://github.com/jclouds/jclouds-karaf/commit/897bd89557b0847075f4fb38a4cb6aab2d5c0bb2