move ruby discovery code into a function

[kbrock]

When you install a new ruby in a different terminal shell, chruby does not pick up on the new ruby version.

The solution is to to source chruby.sh again. Introducing chruby_discover which will allow any shell to pick up on the new ruby versions

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Yes, I was trying to "optimize" this functionality in #492 - but this PR is not an optimization play.

I have had this function defined in my bashrc for a while and thought others may want it as well.