Only chsh to zsh if zsh is not the shell

This PR checks the user's current shell before making the chsh call.  This is a good thing because the chsh call requires a password input, so it'd be nice to avoid that where possible.
I'm not sure what benefits and drawbacks this approach entails. One other approach I could think of is:

```bash
if [[ ps -p $$ | grep zsh ]]; then
```