wiped out all LLVM 3.1 support
reversed macros LLVM_3_6 to LLVM_3_5+ in .cpp and .h files
updated Makefile for reverce LLVM_3_6 flag
fixed broken Makefile (failed to build 3.5)
removed LLVM 3.1 option from python scripts
Merge branch 'master' of https://github.com/ncos/ispc into remove-support-for-old-llvm
remover LLVM 3_1 from cbackend.cpp
Merge pull request #831 from ncos/remove-support-for-old-llvm

Remove support for llvm 3.1