tests pass using bundler, once yaml and multi_json are properly required

When running tests using `bundle exec rake rspec`, the require statement for `yaml` is missing and causing tests to fail. 

The require statement for this library is contained within the `ConfigFile#read_yaml` method; similarly the require statement for `multi_json` is contained in `ConfigFile#read_json`. Since both methods are top-level features, and are fully tested, these require statements should be moved into the main class loader. This will cause no dependency issues as `yaml` has been a standard ruby library since 1.8.7 and `multi_json` is listed as a dependency in both the Gemfile and the gemspec. 

Break the silence, yo.