Revert "Remove stopgap_13632 entirely for now: it doesn't support 2.2.8"

This reverts commit 536d3068.

Gemfile

@@ -37,6 +37,9 @@ gem "rubocop", ">= 0.47", require: false
 # https://github.com/guard/rb-inotify/pull/79
 gem "rb-inotify", github: "matthewd/rb-inotify", branch: "close-handling", require: false
 
+# https://github.com/puma/puma/pull/1345
+gem "stopgap_13632", platforms: :mri if RUBY_VERSION == "2.2.8"
+
 group :doc do
   gem "sdoc", github: "robin850/sdoc", branch: "upgrade"
   gem "redcarpet", "~> 3.2.3", platforms: :ruby