Fix a deprecation warning from rspec.

``` DEPRECATION: include_class is deprecated. Use contain_class instead.
Called from
/home/bob/.rbenv/versions/1.9.3-p392/lib/ruby/gems/1.9.1/gems/rspec-puppet-1.0.1/lib/rspec-puppet/matchers/include_class.rb:7:in
`block (2 levels) in <module:ManifestMatchers>'.
```