update Gemfile to source https://rubygems.org

Running `bundle install` yields a warning about updating the Gemfile. This fixes that warning.
```shell
$ 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.
Fetching gem metadata from http://rubygems.org/.........
Fetching gem metadata from http://rubygems.org/..
Resolving dependencies...
```