Make DangerFile warn instead of error for missing CHANGELOG entry

Dangerfile

@@ -19,7 +19,7 @@ end
 # Request a CHANGELOG entry, and give an example
 has_app_changes = !git.modified_files.grep(/lib/).empty?
 if !git.modified_files.include?('CHANGELOG.md') && has_app_changes
-  fail("Please include a CHANGELOG entry to credit yourself! \nYou can find it at [CHANGELOG.md](https://github.com/CocoaPods/CocoaPods/blob/master/CHANGELOG.md).", :sticky => false)
+  warn("Please include a CHANGELOG entry to credit yourself! \nYou can find it at [CHANGELOG.md](https://github.com/CocoaPods/CocoaPods/blob/master/CHANGELOG.md).", :sticky => false)
   markdown <<-MARKDOWN
 Here's an example of your CHANGELOG entry: