Removing deprecation warnings

A few deprecation warnings popped up when I was trying to use rspec-virtus.  This pull request tries to address those issues. 

```
Forms::Challenges::WeeklyWeightCreateForm has attributes should When you call a matcher in an example without a String, like this:

specify { expect(object).to matcher }

or this:

it { is_expected.to matcher }

RSpec expects the matcher to have a #description method. You should either
add a String to the example this matcher is being used in, or give it a
description method. Then you won't have to suffer this lengthy warning again.
```

```
RSpec::Virtus::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.
```
One thing to note: I believe this line was intended to test `#failure_message` but was calling `negative_failure_message`.  Are the test sufficient?  Should the tests instead check for the full string?