Match strict

Requires https://github.com/tmattia/virtus-matchers/pull/2 to be merged first

Add matcher to check for strict

Usage:

```ruby
class Example
  attribute :terror, String, strict: true
end
```

Spec:

```ruby
it { is_expected.to have_attribute(:terror, String).strict } # passes
it { is_expected.to have_attribute(:terror, String) } # still passes
```