Upgrades rubygems for 1.9.1: 1.3.5 -> 1.3.7
Default Ruby CFLAGS to `-Wno-error=shorten-64-to-32`; don't set `CC`

Ref. #290, #311
Feature detection for `-Wno-error=shorten-64-to-32`
Merge pull request #252 from jessedearing/upgrade_191_rubygems

Upgrades rubygems for 1.9.1: 1.3.5 -> 1.3.7
ruby-build 20130227