Migrate rspec-rails ~> 3.0.

There's still one deprecation. I'll try to handle it in upstream.

```
Deprecation Warnings:

Filtering by an `:example_group` subhash is deprecated. Use the subhash to filter directly instead. Called from /home/ryba/.rvm/gems/ruby-2.1.3/gems/generator_spec-0.9.2/lib/generator_spec.rb:9:in `block in <top (required)>'.
```