Use new stub syntax (to remove deprecation warnings)

When I ran the tests (with rspec 2.99.0), it said:
```
.........

Deprecation Warnings:

Using `stub` from rspec-mocks' old `:should` syntax without explicitly enabling the syntax is deprecated. Use the new `:expect` syntax or explicitly enable `:should` instead. Called from /Users/sean/dev/omniauth-twitter/spec/omniauth/strategies/twitter_spec.rb:83:in `block (4 levels) in <top (required)>'.
```
So I fixed them.