Add GitHub method

I was looking through the documentation and it stated that you could do as follows to install bundles of gems:

```ruby
git 'https://github.com/rails/rails.git' do
   gem 'railties'
   gem 'action_pack'
   gem 'active_model'
end
```

Now, I couldn't help but notice there wasn't a unit test for it. So I made it in 4e33a3f.

I also couldn't help but notice that there was a `#git` method, but no `#github` method. In this PR, I took the time to apply the code and create a test as well. This would allow a developer to write as follows:

```ruby
github 'spree', branch: '2-3-stable' do
  gem 'spree_core'
  gem 'spree_api'
  gem 'spree_backend'
end
```

Instead of:

```ruby
gem 'spree_core', github 'spree', branch: '2-3-stable'
gem 'spree_api', github 'spree', branch: '2-3-stable'
gem 'spree_backend', github 'spree', branch: '2-3-stable'
```

And have those gems as dependencies.