Fix missing ALLOC assignment in rb_monitor_video_modes.

I have no idea. Just no idea.

Fixes #6.
Add check for NULL gamma ramp result (error result).

Returns nil if a ramp isn't returned.
Fix access to ramp pointer ahead of it being acquired.

As far as I can tell, this was an accident when refactoring a while ago
to get better C89 compatibility -- the end result is that a variable
initialization got moved but I forgot to initialize it in the right
place after moving it. D'oh.

Closes #7.
Fix GL module reference from opengl-core in readme.

Previously referred to the 1.x Gl module in opengl-core. This was
renamed to GL (uppercase L) as of 2.x.

(Message updated by @nilium)
Merge branch 'pr5-jstenhouse-readme-fix' into HEAD
Version 0.4.8

- Fix my sloppy refactoring in Monitor#gamma_ramp. (@jstenhouse #7)
- Fix missing ALLOC assignment in rb_monitor_video_modes.
  (@jstenhouse #6).
- Add error checking for getting gamma ramps.
- Update readme according to opengl-core 2.0 naming. (@jstenhouse #5)