Use https://rubygems.org as gems source
To get rid of the following deprecation warning:
```
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.
```
Bump RSpec version and add rspec-its
RSpec needed to be upgraded to void getting this error when installing the dependencies:
```
An error occurred while installing rspec-expectations (2.7.0), and Bundler cannot continue.
Make sure that `gem install rspec-expectations -v '2.7.0'` succeeds before bundling.
```

Adding rspec-it was necessary since rpsec -v2.99.0 extracted `its {}` type of assertion in the process of migrating to Rspec 3.
Convert specs to RSpec 2.99.1 syntax with Transpec

This conversion is done by Transpec 2.3.6 with the following command:
    transpec --keep its

* 51 conversions
    from: obj.should
      to: expect(obj).to

* 44 conversions
    from: == expected
      to: eq(expected)

For more details: https://github.com/yujinakayama/transpec#supported-conversions