Bump guava to a version which works with Java 11

[johanrydstrom]

I work for a client which depends on rdi-client-java to POST data to metamx. In their codebase they also depend on this to work
com.google.common.reflectClassPath.from(ClassLoader.getSystemClassLoader()).getAllClasses()
which stopped working with an upgrade to Java 11. It turns out that this doesn't work (returns an empty Set) in conjunction with older (than 24-something) Guava versions.

Current rdi-client-java (through it's transitive dependency on com.metamx:java-util) drags in com.google.guava:14.0.1. Due to a API-change in Guava (Futures.addCallback now takes 3 parameters, was 2) I can't bump the version in our library; it has to be fixed in rdi-client-java.

I might be able to create a PR, if needed?

[johanrydstrom]

Also in even more modern versions of Guava Futures.transform() has changed it's signature for asynchronous use-cases and should be replaced by Futures.transformAsync().