add lint errors for manifest file

lib/awesim/lint.rb

@@ -23,6 +23,8 @@ module AweSim
       say "Maintenance mode file is missing at public/503.html", :red unless app.path.join("public", "503.html").file?
       say "Gemfile missing rails_12factor gem", :red unless app.bundler_helper.has_gem? "rails_12factor"
       say "Gemfile missing dotenv-rails gem", :red unless app.bundler_helper.has_gem? "dotenv-rails"
+      say "manifest.yml file is missing", :red unless app.manifest.exist?
+      say "manifest.yml file is invalid", :red if app.manifest.exist? && ! app.manifest.valid?
 
       invoke :restart, [], {}
     end