Fix expected table class name in scenario.

This fixes `features/docs/defining_steps/snippets.feature:25` and is alignment
with the changes made in ff3eed83.
Update rspec suite to use rspec 3

This also fixes the `should` usage warnings and some other general ruby syntax
warnings such as

```ruby
expect('My string').to match /string/`
```

to

```ruby
expect('My string').to match(/string/)
```
Upgrade the cucumber suite to work with rspec 3.
Use rspec >= 3.0