Fix gem source warning from Bundler.

Before:

```
$ 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.
```

After: no warning!
Merge pull request #62 from tjschuck/fix_gemfile_source_warning

Fix gem source warning from Bundler.