Fix 'after_cd' hooks triggering invocation.

According to the documentation:
'after_cd (Triggered whenever a user uses the cd command)'

* When rvm is loading this is not a user 'cd' action, so the hooks should
not be triggered.

* Because now 'after_cd' is used only in the `chpwd_funtions` we can simply
move it to `__rvm_cd_functions_set`