Fix Gems search to handle updated Rubygems.org

Rubygems.org went through a redesign lately, and broke this
portion of the workflow.

* Use the Rubygems.org API, instead of parsing HTML search results
* Use HTTPS, as per Rubygems.org docs
* Simplify `Repo::check`
    * Don’t need multiple `return true` conditionals
Merge pull request #44 from jeffbyrnes/fix-gems-search

Fix Gems search to handle updated Rubygems.org