Upgrade Maven compiler plugin.
Bump Proguard versions.

Proguard 5.x finally supports Java 8. We don't compile Getdown to Java 8
bytecode, but Progaurd has to grok the stdlib of the JVM being used to run
Proguard, which previously meant that we couldn't compile Getdown with a Java 8
JVM, which was annoying. Now we can. Yay!