JCLOUDS-415: Preparing for Guava 16.0

Invokable.equals has changed from 15 to 16 so we need to implement the check differently ourselves.

See https://github.com/jclouds/jclouds/pull/265 and https://issues.apache.org/jira/browse/JCLOUDS-415?focusedCommentId=13883073