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.
Merge pull request #3097 from alexpennace/brew_with_yosemite

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