Update Google Guava to version 15.0.

This PR removes the old version of Guava (13.0) that provided Java 1.5 support. This will allow developers make use of a broader spectrum of the library functions.

/cc @sbesson, @joshmoore: Sebastien, I updated .m files that were mentioning the old JAR. Is there anything else I need to do to keep the Matlab code healthy? Would we need to move the Guava JAR closer to client.java so you don't have to code around it?