[Rakefile] Introduce tasks to run specs and generate fixtures

To get the tests running locally, you currently need to figure out how travis is doing it and manually invoke a command to generate the test fixtures. To make this a bit easier, I've moved the logic into the Rakefile so you can run `bundle exec rake` to generate the fixtures and run the specs.

```bash
$ bundle exec rake -T
rake build              # Build slather-1.0.1.gem into the pkg directory
rake generate_fixtures  # Generate the test fixtures
rake install            # Build and install slather-1.0.1.gem into system gems
rake release            # Create tag v1.0.1 and build and push slather-1.0.1.gem to Rubygems
rake specs              # Run all the specs
```