Update the dependency on Guava (r06 -> 15.0)

Hi,

Here is a patch upgrading the dependency on Guava. The latest release removed the `MapMaker.makeComputingMap()` method used by Stapler. The patch follows the migration guide published by the Guava team:

http://code.google.com/p/guava-libraries/wiki/MapMakerMigration
http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/cache/CacheBuilder.html

Thank you