Improve Application Security

* Upgrade to the latest patchlevel of Ruby 2.0 includes security fixes [1]
* Upgrade to the latest versions of the gems the app depends on (not strictly security, but good practice)
* Switch to secure URLs for downloading gems from GitHub (per bundle-audit report)

[1]:https://www.ruby-lang.org/en/news/2013/11/22/ruby-2-0-0-p353-is-released/