Use a warning instead of a failure

lib/cocoapods/validator.rb

@@ -400,7 +400,7 @@ module Pod
       return if spec.source.nil? || spec.source[:http].nil?
       url = spec.source[:http]
       return if url.downcase.start_with?('https://')
-      error('http', "The URL (#{url}) doesn't use the encrypted HTTPs protocol. " \
+      warning('http', "The URL (#{url}) doesn't use the encrypted HTTPs protocol. " \
               'It is crucial for Pods to be transferred over a secure protocol to protect your users from man-in-the-middle attacks. '\
               'Please update the URL to use https and try again.')
     end