v1.1.4

• Add ruby 2.1.4 and 2.1.5
• Update machete instructions in README
  (https://www.pivotaltracker.com/story/show/82692644)
• Test regression around copying the same file to itself
  (https://www.pivotaltracker.com/story/show/81613240)
• Add a regression test for stack env variable
  (https://www.pivotaltracker.com/story/show/82289072)
• Merge upstream changes
  (https://www.pivotaltracker.com/story/show/82765888)
  • Include fix to unbounded cache size
    (heroku/heroku-buildpack-ruby#314)
  • Show full stack trace on all unexpected errors
    (heroku/heroku-buildpack-ruby@b84422c)
  • Suppress backtrace from likely errors
    (heroku/heroku-buildpack-ruby@74cba7c)