Changed oh-my-zsh submodule to use https script

For some reason, Vagrant machines keep failing to clone this:

Host key verification failed.
fatal: Could not read from remote repository.
Please make sure you have the correct access rights and the repository exists.
Clone of 'git@github.com:robbyrussell/oh-my-zsh.git' into submodule path 'zsh/bundle/oh-my-zsh' failed