Add LLVM 3.2/3/4 to Travis CI.

Changes the Travis CI configuration to use the following packages:
- LLVM 3.1 from precise
- LLVM 3.2 from precise-proposed
- LLVM 3.3 and 3.4 from llvm.org snapshot

LLVM 3.4 builds are allowed to fail because this is a development
snaphot.
Reorganizes also the cmake command line.