Fix rspec 3.0 deprecation warnings

This pull request tries to fix several rspec deprecation warnings including:

* Using the new ```be_falsey``` and ```be_truthy``` conditionals.
* Switch from ```pending``` to ```skip``` in order to keep old behavior on Rspec 3
* Use expectations to avoid 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)>'.
```

* Use ```expects()```  to avoid ```its()``` deprecation warnings.
* Using ```RSpec::Mocks::Double``` instead of deprecated ```RSpec::Mocks::Mock```

Verification
-----------
- [ ] Be sure travis is green
- [ ] Run specs locally to ensure there aren't rspec deprecation warnings