[C] Update gson dependency version to 2.2.4. Fixes BUKKIT-4848.

#### Description:  
##### The Issue:
The gson dependency in CraftBukkit is outdated. It is currently at 2.1, while the latest version is 2.2.4.

##### Justification for this PR:
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 is for the server administrator to 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.

##### PR Breakdown:
This PR updates the Gson dependency to 2.2.4 in the pom.xml
 
##### Testing Results and Materials:
I successfully compiled CraftBukkit, ran a server with the compiled jar, and ensured that CraftBukkit could successfully retrieve a CraftBukkit artifact (to ensure that the gson features in CraftBukkit were not broken).
 
##### JIRA Ticket: 
BUKKIT-4848 - https://bukkit.atlassian.net/browse/BUKKIT-4848