Add alias to Matcher for RSpec v3.0

The following RSpec:

```ruby
expect(registration).to have_valid(:social_security_number).when '123-45-6789', '123456789'
```

produces this warning when used with RSpec v3.0:

```
ValidAttribute::Matcher implements a legacy RSpec matcher
protocol. For the current protocol you should expose the failure messages
via the `failure_message` and `failure_message_when_negated` methods.
```

This PR simply creates an alias to silence this warning.