Add sd_notify gem to allow puma to notify systemd about readiness

This adds an optional gem sd_notify, which, when available, allows puma
to notify systemd about having finished its startup. This allows puma to
be Type=notify instead of Type=simple in its service file.

Gemfile

@@ -186,6 +186,7 @@ gem 'rack-timeout', '~> 0.5.1', require: 'rack/timeout/base'
 group :puma do
   gem 'puma', '~> 5.3.1', require: false
   gem 'puma_worker_killer', '~> 0.3.1', require: false
+  gem 'sd_notify', '~> 0.1.0', require: false
 end
 
 # State machine

Gemfile.lock

@@ -1153,6 +1153,7 @@ GEM
       addressable (>= 2.3.5)
       faraday (> 0.8, < 2.0)
     scientist (1.6.0)
+    sd_notify (0.1.0)
     securecompare (1.0.0)
     seed-fu (2.3.7)
       activerecord (>= 3.1)
@@ -1605,6 +1606,7 @@ DEPENDENCIES
   rugged (~> 1.1)
   sanitize (~> 5.2.1)
   sassc-rails (~> 2.1.0)
+  sd_notify (~> 0.1.0)
   seed-fu (~> 2.3.7)
   selenium-webdriver (~> 3.142)
   sentry-raven (~> 3.1)