Fail if yarn is outdated (#363)

* Fail if yarn is outdated

* Revert other changes

lib/tasks/webpacker/check_yarn.rake

@@ -1,11 +1,14 @@
 namespace :webpacker do
   desc "Verifies if yarn is installed"
   task :check_yarn do
+    required_yarn_version = "0.20.1"
+
     begin
-      version = `yarn --version`
-      raise Errno::ENOENT if version.blank?
+      yarn_version = `yarn --version`
+
+      raise Errno::ENOENT if yarn_version.blank? || Gem::Version.new(yarn_version) < Gem::Version.new(required_yarn_version)
     rescue Errno::ENOENT
-      puts "Webpacker requires yarn. Please download and install Yarn https://yarnpkg.com/lang/en/docs/install/"
+      puts "Webpacker requires Yarn version >= #{required_yarn_version}. Please download latest version and install https://yarnpkg.com/lang/en/docs/install/"
       puts "Exiting!" && exit!
     end
   end