Bootstrap.sh check for git repository before cloning

This commit simply checks the directory for a `.git` folder before cloning the git repository. 

This stops people like me who forked and cloned the repo before running
```
curl https://raw.githubusercontent.com/codecombat/codecombat/master/scripts/devSetup/bootstrap.sh | bash -s your_repository_url
```
 from ending up with two copies of the code in the same directory.