Fixing `resolve` entry point.

`package.json` lists the main entry point as `grunt.js`, which does not exist within the package. This is breaking `resolve( )`, as the lookup fails due to a not-found file. Changing `package` to `Gruntfile.coffee`, which is the actual file name, fixes the issue.

Though Grunt has no need for `resolve`, the lookup is used by some Grunt plugins, such as LinemanJS, to identify nested plugins and Grunt tasks.