If Papertrail is causing a problem, try a different add on. I’ve been using LogEntries without much of a problem. https://addons.heroku.com/#logging
Also try to lower your Unicorn worker processes so it uses lower total memory. Instead of the default of 3 (per box/dyno), try 2.
https://devcenter.heroku.com/articles/rails-unicorn#unicorn-worker-processes
You can also run a memory profiler on your app:
http://timetobleed.com/memprof-a-ruby-level-memory-profiler/
https://www.ruby-toolbox.com/search?utf8=%E2%9C%93&q=profile