Move Achilles to Guava 15

Guava used by Achilles is too old (12). It is not longer compatible with latest JDK 1.7.0_51-b13 https://code.google.com/p/guava-libraries/issues/detail?id=1635

However, can't move yet to Guava 16.0.1 because of this issue in Cassandra https://issues.apache.org/jira/browse/CASSANDRA-6639 so can't verify that all unit tests are passing (it just hangs on my machine)

As a workaround, changed Guava version to 15.0. Tests are passing now, and all projects that use Achilless still can use Guava 16.0.1 by overriding it in Maven (or whatever tool is used).