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.

lib/capistrano/tasks/restart.cap

@@ -1,11 +0,0 @@
-namespace :deploy do
-  desc "Restart Unicorn"
-  task :restart do
-    on roles(:app) do
-      execute "kill -QUIT `cat #{deploy_to}/pids/unicorn.pid`; true"
-      execute "kill -QUIT `cat #{shared_path}/pids/unicorn.pid`; true"
-      execute "kill -QUIT `cat #{shared_path}/tmp/pids/unicorn.pid`; true"
-      execute "cd #{release_path} && bundle exec unicorn -c config/unicorn.rb -E production -D"
-    end
-  end
-end