Silence gem list command output

Prevents `appraisal rake` to write a boolean on the standard output when
checking the installed version of bundler

Bug introduced in thoughtbot#168 5868643

Fix: thoughtbot#180

lib/appraisal/command.rb

@@ -42,7 +42,7 @@ def with_clean_env
 
     def ensure_bundler_is_available
       version = Utils.bundler_version
-      unless system %(gem list -i bundler -v #{version})
+      unless system %(gem list --silent -i bundler -v #{version})
         puts ">> Reinstall Bundler into #{ENV["GEM_HOME"]}"
 
         unless system "gem install bundler --version #{version}"