Add missing Gemfile

Author: radar

gemfiles/Gemfile.rails-6.0.x

@@ -0,0 +1,13 @@
+source 'https://rubygems.org'
+
+gemspec :path => '..'
+
+gem 'activesupport', '~> 6.0.0'
+gem 'mocha', '~> 1.7.0'
+gem 'test_declarative', '0.0.6'
+gem 'rake'
+gem 'minitest', '~> 5.1'
+
+platforms :mri do
+  gem 'oj'
+end