Fixed passing of NODE_ENV setting to Express

The line:

           service = child_process.spawn('node', [options.file], {
                NODE_ENV: options.env
            });

did not actually set NODE_ENV to options.env.

This does work:

            process.env.NODE_ENV = options.env;
            service = child_process.spawn('node', [options.file]);

Third parameter of child_process.spawn is not required because it defaults to process.env.
Merge pull request #6 from erichosick/master

Fixed passing of NODE_ENV setting to Express