refactoring depends homebrew-hhvm

- gcc48 depends only for build source code (useful binutilsfb).
- current stable jemalloc v3.6.0
- sort depends, remove duplicate depends
- README
 - 1.) Instructions to uninstall tap
 - 2.) Sort OSX version
 - 3.) Add «Known Problems»
- hhvm: GCC 4.9 Release stable