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 surprising XZ compression for the clang-3.4 tarball!)
- default tarball is Clang 3.4 for Ubuntu-12.04 64bit
- new attribute `download_url` introduced as LLVM packaging is pretty
  inconsistent across releases (no stable naming *conventions*, no stable compression *agreement*)

Note also that I removed the details about 32/64 bit supported architectures in README (for sake of simplicity).