Update package.json with correct coffee-script version

As mentioned in [this issue](https://github.com/rstacruz/js2coffee/issues/199) versions of coffee-script after 1.1.0 cause an error in this script. Workaround proposed in the issue (setting coffee-script dependency in `package.json` to `= 1.1.0` instead of `>= 1.1.0` solved the issue nicely.