Replace `source :rubygems` with url for rubygems.org

Bundler (>= 1.3.0) says:
```
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.
```
Merge pull request #169 from tricknotes/suppress-bundler-warning

Replace `source :rubygems` with url for rubygems.org