Improved listing output

Just a quick modification to the output format so that it looks the same as the one used by `pod search`.

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Notes:

1. This is my very **first pull request on ruby code** (I'm a ruby beginner), so please review!
2. I was **unable to execute `rake` to run the RuboCop tests** before submitting this PR (probably to sthg obscure specific to my ruby environment?)… It didn't seem to me that the tests were to be adapted to the new output format but I couldn't test it . I'll wait for Travis to tell me!
3. But I **successfully installed the gem** (`gem build cocoapods-plugins.gemspec && sudo gem install cocoapods-plugins-0.1.1.gem`) and **successfully executed both `pod plugins` and `pod plugins --verbose` commands** and everything seemed to work as expected.