Fix rubygems timeout.rb segfaults on REE

Fixes #80, https://github.com/sstephenson/ruby-build/wiki#187-p302-and-lower-segfaults-for-https-requests-on-os-x-107
Use gcc 4.8 again on Mavericks for ree & 1.8.7

The gcc-4.8 compiler works properly for these rubies on Mavericks, and
we want to minimize dependencies on apple-gcc-42 because it's a
maintenance nightmare.
Merge pull request #115 from boxen/fix-ree-builds

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