Switch default homebrew gcc to 4.8 for OS X Yosemite (and beyond)

Currently, gcc 4.6 cannot compile on OS X 10.10 without patching.

Fixes #3096.

I evaluated adding support for gcc 4.9, but one of the Ruby versions I tried (1.9.2-p318) will not build with that compiler. I'm not sure if teaching RVM 1.x to use an older compiler for older Ruby versions is practical. In any case, gcc 4.8 is still a current, supported gcc version.