Fixing version to 3.1.1

/usr/bin/hhvm --version
HipHop VM 3.1.1 (rel)
Compiler: heads/HHVM-3.1-0-g1e5e958bf6ad47a3075e74497b63d18d3cc9112a
Repo schema: e6c40bcbcd0490c897a393af15309661f2a1de9e