fix haml linter declaration

This is necessary in order to boot delayed_job -_-
Without this, it attempts to inherit a missing class

lib/linters/application_name_linter.rb

@@ -1,3 +1,4 @@
+if defined?(HamlLint)
   module HamlLint
     class Linter::ApplicationNameLinter < Linter
       include LinterRegistry
@@ -44,3 +45,4 @@ module HamlLint
       end
     end
   end
+end