Use new expectation syntax when using rspec

[Rspec has a new expectation syntax](http://myronmars.to/n/dev-blog/2012/06/rspecs-new-expectation-syntax):

```ruby
it { something.should be_valid }

# vs

it { expect(something).to be_valid }
```

... which is recommended by rspec devs. It simplifies monkey-patches in rspec and would be default option in rspec 3 (old syntax will be probably disabled).

You can read more on [rspec dev blog](http://myronmars.to/n/dev-blog/2013/07/the-plan-for-rspec-3#what_about_the_old_expectationmock_syntax).