Changing the ZSH theme to gentoo

Gentoo is a simple theme, with a prompt that looks a lot like the default bash prompt.
It shows the username and hostname, which is very useful when you connect to the box
using ssh.
Fixes the regex ins the ZSH plugins configuration task

 The regex was looking for a line starting with "plugins=(git)" (the default configuration),
 making it match the only in the first run.