Release Name: 1.3.1

Notes:
New release of Ruby/SDL. 


Changes:
Fix the bug that we cannot compile when SMEPG is installed but SDL_mixer
 is not installed.(1.2.0's change is incomplete)
Add following methods
  SDL::Surface#Rloss
  SDL::Surface#Gloss
  SDL::Surface#Bloss
  SDL::Surface#Aloss
  SDL::Surface#Rshift
  SDL::Surface#Gshift
  SDL::Surface#Bshift
  SDL::Surface#Ashift
  SDL::Surface#pitch
  SDL::Surface#bytes_per_pixel
Fix following methods
  SDL::Surface.pixels