Added git and zsh detection before installing.

Hi!

I added git and zsh detection before installing oh-my-zsh.

This detection might sound redundant, but is actually useful because it happened to me too many times, to install oh-my-zsh on a clean vps, before installing git and zsh. In this particular case, the script always outputs "oh-my-zsh installed." which is not correct.


Cheers!