Update capistrano's restart unicorn task

Our current unicorn task wasn't working in some cases. We also had a
version in the `capistrano` branch, which was the one we recommended.
However, that version assumed RVM, a certain ruby version and a certain
deploy folder were used. This version uses `bundle exec` and variables
like `release_path`, so it does not depend on any specific
configuration.

Even if we're replacing unicorn with puma, I wanted to make this change
in case we need it as a reference in the future.

lib/capistrano/tasks/restart.cap

@@ -1,10 +1,11 @@
 namespace :deploy do
-  desc 'Commands for unicorn application'
-  %w(start stop force-stop restart upgrade reopen-logs).each do |command|
-    task command.to_sym do
-      on roles(:app), in: :sequence, wait: 5 do
-        execute "/etc/init.d/unicorn_#{fetch(:full_app_name)} #{command}"
-      end
+  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