Fix for #14: infinite loop in authenticate

Using `connection.post` avoids an infinite loop where `authenticate` calls `post` for authentication and `post` calls `authenticate` due to missing authentication.
Already merged, but there's a nit-picky style issue here. You should prefer leaving off the trailing comma form the last key/value pair or value in collections.

See the [Ruby style guide](https://github.com/bbatsov/ruby-style-guide/blob/master/README.md#collections):

> Avoid comma after the last item of an Array or Hash literal, especially when the items are not on separate lines.