Use sed for *PATH stripping

This is an attempt of replacing shell string replacement with sed.

Also replaced some logic, I now always strip any version from the different `*PATH`'s and prepending the new one, where previously the version was replaced if available and otherwise prepended. Or at least, that was what I could make of it.

Should fix #407 #393 #392  

~~Although I think that we could make some kind of `nvm_strip` function to prevent this much code duplication.~~ Done as `nvm_strip_path $PATH /bin`