README is outdated; update llvm to 3.7

[duncancarroll]

Spent a while trying to compile this against llvm35 as per the OS X instructions, when it turns out cmake will fail if the version < 3.7. It's (as of 8/1) unreleased so git repo for source download is in a branch here: https://github.com/llvm-mirror/llvm/tree/master Hopefully this saves someone some time.

[chfast]

README updated in develop branch (now the default one).