v1.3.1

• Update provided MRI ruby versions for CVE-2015-1855 (https://www.pivotaltracker.com/story/show/92437386)
  • 2.0.0p645
  • 2.1.6
  • 2.2.2
• Document Contribution guidelines (https://www.pivotaltracker.com/story/show/92459188)
• Do not use cached dependencies when a running app is restaged on a different rootfs (https://www.pivotaltracker.com/story/show/91630536)
• Merge upstream changes from Heroku up to SHA f349dfc (https://www.pivotaltracker.com/story/show/90537738)
• Add support for jruby versions (https://www.pivotaltracker.com/story/show/90537738)
  • jruby-1.7.19:
    • ruby-1.8.7
    • ruby-1.9.3
    • ruby-2.0.0
  • jruby-9.0.0.0:
    • ruby-2.2.0