Fix REE build flags, and use gcc-4.8 by default

This builds on #112, but reverts the switch to gcc 4.2.

I've successfully built ree using gcc 4.8 on both Mavericks and Yosemite. I believe the 4.8 issues reported in #112 were related to a bad 4.8 compiler rather than bugs building Ruby.

See https://github.com/sstephenson/ruby-build/issues/488 for the source of the updated `CFLAGS`

Fixes #80 

cc @grosser @rafaelfranca