Log Message:
Update ruby-ffi-yajl to 2.2.2.

2.2.2 (07/15/2015)

Bugs fixed

* Fix uninitialized constant StringIO error in C extension

2.2.1 (07/13/2015)

Bugs fixed

* Aaron Lasseigne: Thanks for fixing my bad .travis.yml
* Kouhei Sutou: Numerous circular argument reference fixes, method redefined,
  assigned but unused warnings, superclass mismatch warnings, etc.
* Fixed uninitialized variable when using C-ext.
* Fixed README require
* Fixed superclass mismatch in rbx when using C-ext.
* Lots of rubocop fixes, and added rubocop to travis