Fix rspec-collection_matchers specs deprecation warnings

There are several rspec 3 deprecation warnings related to collection matches, like the next one: 

```
`expect(collection).to have_at_least(1).address` is deprecated. Use the rspec-collection_matchers gem or replace your expectation with something like `expect(collection.length).to be >= 1` instead. Called from /metasploit-framework/spec/lib/rex/socket/range_walker_spec.rb:18:in `block (4 levels) in <top (required)>'.
```

This pull request tries to solve these warning by using the recommended syntax instead of using the   rspec-collection_matchers gem.

Requirements
--------
- [ ] Worths to land #3711 first to avoid conflicts here

Verification
-------
- [ ] Be sure travis is green!