Change the default -j of install_llvm to 2

Perhaps we will run this script in local machines, Travis CI servers
and/or Docker hub servers. We do not want to eat up their CPU.
Only use GCC in Travis CI server

It seems that there is a bug of the clang installed in Travis CI.
Lacking of clang test is OK. Anyway, we build our LLVM by ourselves.
Let's suppose the world begins with NIL.
Merge branch 'master' of github.com:liuml07/giri