Replace :rubygems in Gemfile with 'https://rubygems.org'.

Bundler some time ago started display warning message like follow:

```sh
$ bundle install

The source :rubygems is deprecated because HTTP requests are insecure.
Please change your source to 'https://rubygems.org' if possible, or 'http://rubygems.org' if not.
...
```

So, this thange is to remove it and let everything go smother :)

(I'm using bundler version 1.3.5)

Please read more:

* [bundler commit(bc4403b): warn on deprecated symbol source](https://github.com/bundler/bundler/commit/d30026e9c8fc6c98478120866a47ca5b619251b8)