Log Message:
Update ruby-SDL to 2.2.0.

version 2.2.0
Add following constants
  (thanks to oscdir for SDL::GL::*, thanks to Rafael Sevilla for SDL::Event::*)
* SDL::Event::APPMOUSEFOCUS
* SDL::Event::APPINPUTFOCUS
* SDL::Event::APPACTIVE
* SDL::GL::STEREO
* SDL::GL::MULTISAMPLEBUFFERS
* SDL::GL::MULTISAMPLESAMPLES
* SDL::GL::ACCELERATED_VISUAL
* SDL::GL::SWAP_CONTROL
Add following methods(thanks to soutaro)
* SDL::TTF#hinting
* SDL::TTF#hinting=
Rename  SDL::Mixer::Wave.destroyed_ to SDL::Mixer::Wave.destroyed? (thanks to \ 
akicho8)

version 2.1.3.1
Change the name of the option for bundled SGE
Change the way of workaround for -Dmain=SDL_main on Windows platform

version 2.1.3
Fix following methods
* SDL::TTF#draw*
Update documents
Fix compilation problem on FreeBSD(Thanks Rusmir Dusko)
Fix codes about multi-thread
Bundle SGE(Please do not use this bundled SGE if you have other way to use SGE)

version 2.1.2
Improbe the handling of strings
Fix following methods
* SDL::Surface#draw_bezier
Update documents