Update gson dependency version to 2.2.4. Fixes BUKKIT-4847.

The gson dependency in CraftBukkit is outdated. It is currently at 2.1, while the latest version is 2.2.4.

There are many plugins which rely on gson features only available in version 2.2+

As shading other versions of gson is not allowed for BukkitDev plugin submissions, developers who wish to use these features are relegated to forcing the user to use a classpath-based server launch script. This is necessary as the only other way to use the newer versions of gson the server administrator must create a lib/ folder to host the latest version.

Updating the gson version will result in a greater range of options for developers who require serialization, while not requiring the server administrator to perform unnecessary actions.