Fix sed usage errors in jumpstart.sh

I've run following command  entered d7_test as a project name and empty string as a project repo url:
```
vadim@vjdesktop:/var/www$ sh -c "`curl https://raw.github.com/blinkreaction/drupal-starter-repo/master/scripts/jumpstart.sh`"
```

It looks like modules were downloaded successfully, but eventually I've seen following error:
```
Renaming 'starter' to 'd7_test'
sed: can't read s/starter/d7_test/g: No such file or directory
Renaming failed, aborting.
```

This should be fixed by using `sed -i "..." <filename>` instead of `sed -i "" "..." <filename>`.