Update for LLVM 3.2

This pull request updates LLVM version to 3.2.

There is an open discussion whether invoking llvm-config with version suffix is a correct way to configure it. 

 * Homebrew does not support version slots (i.e. multiple simultaneously installed versions of a package) by design.
 * Debian and derived distributions do.
 * LLVM API is unstable and often changes. ruby-llvm should check if the version of LLVM is supported, and select a supported one if several are available.

See also https://github.com/jvoorhis/ruby-llvm/pull/20.