Premailer itself doesn't support Ruby < 2.1 at all.

akzhan · web-flow · commit 7f1774ad9e08 · 2017-03-08T01:17:51.000+03:00
diff --git a/Gemfile b/Gemfile
@@ -6,10 +6,4 @@ gemspec
 
 group :development, :test do
   gem "webmock"
-
-  unless ENV['TRAVIS']
-    gem 'ruby-debug', :platform => :mri_18
-    gem 'ruby-debug19', :platform => :mri_19, :require => 'ruby-debug'
-  end
 end
-
