Put chruby-global.sh in chruby root rather than env.d

I ran into a problem where `chruby-global.sh` was being sourced before `chruby.sh`.

This caused bash to print an error along the lines of `chruby: command not found` whenever I opened a new terminal.

Instead of relying on implicit order dependencies, I've moved `chruby-global.sh` into the chruby root and sourced it from `chruby.sh` in env.d