Add missing Gemfile

gemfiles/activerecord_5.2.2.gemfile

@@ -0,0 +1,20 @@
+# This file was generated by Appraisal
+
+source "https://rubygems.org"
+
+gem "activerecord", "~> 5.2.2", require: "active_record"
+gem "activesupport", "~> 5.2.2", require: "active_support/all"
+gem "actionpack", "~> 5.2.2", require: "action_pack"
+
+platforms :jruby do
+  gem "activerecord-jdbcsqlite3-adapter"
+  gem "jdbc-sqlite3"
+  gem "jdbc-postgres"
+end
+
+platforms :ruby, :mswin, :mingw do
+  gem "pg", "~> 1.3.4"
+  gem "sqlite3", "~> 1.7.3"
+end
+
+gemspec path: "../"