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
Merge pull request #81 from denji/refactoring

refactoring depends homebrew-hhvm