Update paddle classic verifier to match their docs

Closes #1199

Author: excid3

lib/pay/paddle_classic/webhooks/signature_verifier.rb

@@ -38,8 +38,8 @@ def verify
           data_serialized = serialize(data_sorted, true)
 
           # verify the data
-          digest = OpenSSL::Digest.new("SHA1")
-          pub_key = OpenSSL::PKey::RSA.new(public_key)
+          digest    = OpenSSL::Digest::SHA1.new
+          pub_key   = OpenSSL::PKey::RSA.new(public_key).public_key
           pub_key.verify(digest, signature, data_serialized)
         end