Update several build plugins, Gson, and Netty

[calcastor]

PGM uses Adventure for several features, as does the dependent project Community. Recent changes to Adventure result in a dependency on Gson 2.7+. Given that the latest version of Gson is compatible with Java 7 and above, let's just update to that instead.

Some build plugins are updated due to build issues encountered with the shade plugin and latest Gson. These actually seem to help reduce the size of the final jar.

Also update to latest Netty.