Add rake as test, dev dependency

This fixes the broken Travis CI build:
https://travis-ci.org/rubymotion/teacup/builds/7448079

Which was caused by a lack of rake in the gemfile while using bundle exec:

```
bundle exec rake spec
/Users/travis/.rvm/gems/ruby-1.9.3-p392@global/gems/bundler-1.3.5/lib/bundler/rubygems_integration.rb:214:in `block in replace_gem': rake is not part of the bundle. Add it to Gemfile. (Gem::LoadError)
	from /Users/travis/.rvm/gems/ruby-1.9.3-p392@global/bin/rake:18:in `<main>'
	from /Users/travis/.rvm/gems/ruby-1.9.3-p392/bin/ruby_noexec_wrapper:14:in `eval'
	from /Users/travis/.rvm/gems/ruby-1.9.3-p392/bin/ruby_noexec_wrapper:14:in `<main>'
```