Fix gnzh theme to detect local rvm installations

gnzh rvm detection only detects system wide installations. I've fixed it so it now detects local installs too.

Since themes is loaded fairly early in `.zshrc`, rvm loading scripts and user path additions are usually aren't run until near the end. As a result, even though `rvm-prompt` works just fine in an interactive shell it's not detected when the theme is loaded.