Explicitly include Rake::DSL to avoid warnings

Not sure if this is the right solution, but I saw some warnings when running my specs that use Fabricate:

```console
WARNING: Global access to Rake DSL methods is deprecated.  Please include
    ...  Rake::DSL into classes and modules which use the Rake DSL methods.
WARNING: DSL method Module#desc called at /home/maciek/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/fabrication-2.11.3/lib/tasks/defined_fabricators.rake:2:in `block in <top (required)>'
WARNING: DSL method Module#task called at /home/maciek/.rbenv/versions/2.1.2/lib/ruby/gems/2.1.0/gems/fabrication-2.11.3/lib/tasks/defined_fabricators.rake:3:in `block in <top (required)>'
```

Editing the file to add `include Rake::DSL` seems to fix things.