Revert "Revert "Change toRaw() to rawValue for Xcode6.1-beta2.""

This reverts commit e2cb937748d34da20b10d26fd8a9ab7a9f5ba045.
Fix adopting NilLiteralConvertible for Xcode6.1-GM.
Merge pull request #7 from inamiy/Xcode6.1-GM

Fix code for Xcode6.1-GM.