Do not set PATH on in rvm function

[JHK]

As $GEM_HOME is set afterwards this will cause /bin to be included in the PATH. This caused for me messing up commands I had in /usr/local/bin to be preferred over /bin:

➜  ~ which bash
/usr/local/bin/bash
➜  ~ rvm default
➜  ~ which bash
/bin/bash

This comes from https://rvm.io/integration/fish