boost 1.49.0: Fix for linuxbrew

`boost1.49` is required for cufflinks and probably many packages out there.
See https://github.com/Homebrew/homebrew-science/pull/1492
We shouldn't have `if OS.mac?` in formulae; 99.9% of users of Homebrew are on OSX and the others should be using Linuxbrew and handling these cases there.