Make guava an optional dependency of java-driver-guava-shaded

[tolbertam]

With CASSJAVA-52, the java-driver-guava-shaded module is now in tree.

This appears to work great, but there is a slight issue with the dependency tree that allows unshaded guava packages to be imported within the project.

It looks like marking guava as an optional dependency in java-driver-guava-shaded resolves this.

patch by Andy Tolbert; reviewed by for CASSJAVA-76

[netudima]

LGTM (nb)

[absurdfarce]

Apologies, gents, I missed this one. I'll merge this one now and fix up the commit msg after the fact.