Version 1.5.2

Remove duplicated version constants.

The gemspec and main snow-math.rb now both require the version script
that defines the version constant. Should stop me from forgetting to
update one or the other in the future.
Change neardouble to two_near in mat4_frustum.

Makes more sense as a variable name. Small change, but noticed it while
going through the code.
Fix mat4_perspective output.

The perspective matrix was off by quite a bit in odd cases and I wasn't
quite sure why 'til I looked at this and compared it to gluPerspective's
matrix, at which point it became obvious that I must've been completely
out of it when I wrote the perspective function.

So yeah, that's fixed.
Version 1.5.3

Fixes mat4_perspective output (so Mat4::perspective will return correct
results now).