Add Rake

`bundle exec rake` すると rake がないよって怒られるので、Gemfile に含めておきたい。

```
% bundle exec rake
/Users/megurine/.rbenv/versions/2.1.0/lib/ruby/gems/2.1.0/gems/bundler-1.6.0/lib/bundler/rubygems_integration.rb:240:in `block in replace_gem': rake is not part of the bundle. Add it to Gemfile. (Gem::LoadError)
	from /Users/megurine/.rbenv/versions/2.1.0/bin/rake:22:in `<main>'
```