Add custom spring command

From `spring 0.9.1`,`spring testunit`  has moved to [spring-commands-testunit](https://github.com/jonleighton/spring-commands-testunit). Now, add custom spring command to works with rails >= 4.0 and spring >= 0.9.1

````ruby
guard :minitest, spring: 'rake test' do
  # ...
end
```