Upgrade ruby, gems

[kgrz]

• Upgrade nokogiri dependency

  Avoids the following vulns:

  CVE-2017-9050 CVE-2017-18258

• Upgrade sinatra gem to latest 1.x

  This upgrades the rack gem to one that doesn't have a security vulnerability CVE-2015-3225

  https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2015-3225

• Upgrade ruby

  Ruby 2.2 is EOLed. Heroku supports 2.5.1, so using that.