Add rspec-its gem

This PR will fix following error on spec.


```shell-session
$ bundle exec rake spec
/Users/mizoR/.rbenv/versions/2.1.1/bin/ruby -I/Users/mizoR/.ghq/github.com/mizoR/emojimmy/.bundle/bundle/ruby/2.1.0/gems/rspec-core-3.0.2/lib:/Users/mizoR/.ghq/github.com/mizoR/emojimmy/.bundle/bundle/ruby/2.1.0/gems/rspec-support-3.0.2/lib -S /Users/mizoR/.ghq/github.com/mizoR/emojimmy/.bundle/bundle/ruby/2.1.0/gems/rspec-core-3.0.2/exe/rspec spec/emojimmy/mixin_spec.rb spec/emojimmy_spec.rb
/Users/mizoR/.ghq/github.com/mizoR/emojimmy/spec/emojimmy/mixin_spec.rb:37:in `block (5 levels) in <top (required)>': undefined method `its' for #<Class:0x007fd80363e830> (NoMethodError)
```

> * Extract `its` support to a separate gem. (Peter Alfvin)
>
> ref: https://github.com/rspec/rspec-core/blob/master/Changelog.md#300beta1--2013-11-07