Revert "upgrade to guava 17.0." until geoserver upgrades.

This reverts commit 4de7ffcd3253dd4043f44a97d4f24409e026a14f
in order not to cause a compile error on geoserver master due
to guava 17 being dragged as a transitive dependency and override
geoserver's version.
The patch can be re-applied once geoserver upgrades to guava 17.0 too.