Pull upstream changes from Travis CI (tarball)

Changes in tarball recipe:

- Ark cookbook is used to download and deploy the tarball (0.8+ is
  required because of XZ compression of clang-3.4 tarball!)
- default tarball is Clang 3.4 for Ubuntu-12.04 64bit
- attribute download_url introduced as LLVM packaging is pretty
  inconsistent across releases (no naming conventions, XZ or GZ
  compression)
Merge pull request #2 from michaelklishin/clang-3.4-with-ark-dep

Pull upstream changes from Travis CI (tarball)