Add back Signer.use_crt? to fix backwards compatability break. (#3108)

Author: alextwoods

gems/aws-sigv4/lib/aws-sigv4/signer.rb

@@ -779,6 +779,13 @@ def presigned_url_expiration(options, expiration, datetime)
       end
 
       class << self
+
+        # Kept for backwards compatability
+        # Always return false since we are not using crt signing functionality
+        def use_crt?
+          false
+        end
+
         # @api private
         def uri_escape_path(path)
           path.gsub(/[^\/]+/) { |part| uri_escape(part) }