Small improvements on module

Hi Edoardo,

Was looking for a oh my zsh module and saw you opened a PR to add yours on boxen organzation, so instead of create another one, made some small improvements and now sending the PR

Small list, each separated in a different commit
* Bumped gem versions so we can run script/cibuild
* Added check so install script is run only if theres no ~/.oh-my-zsh folder (remove directory and re-run boxen to reinstall)
* Updated script install URL based on oh-my-zsh README

Hope you enjoy it.

See you,
Celso