Use puma instead of unicorn

Puma is the server we use in the development environment, so this way we
don't need to maintain two servers. Furthermore, puma seems to offer a
few advantages over unicorn (like multithreading) and no disadvantages.

Capfile

@@ -12,6 +12,9 @@ require "capistrano/delayed_job"
 require "whenever/capistrano"
 require "rvm1/capistrano3"
 
+require "capistrano/puma"
+install_plugin Capistrano::Puma
+
 #SCM: Git
 require "capistrano/scm/git"
 install_plugin Capistrano::SCM::Git