Update Google Gson to 2.2.4.

Only contains a single bugfix over 2.2.3, but may as well keep up to date.
Commit comment amended to follow http://git-scm.com/docs/git-commit style
as requested by #flerda
Merge pull request #152 from donald-w/v2.1-dev-gson-2.2.4-upgrade

Update Google Gson to 2.2.4