Support for newer PGP public key servers

[dbarranco]

Hello!

Thanks for the work done here, it's pretty useful :)

I was wondering if the hkp library could be updated to support newer public keyservers, like https://keys.openpgp.org/as, for me, it seems to be more reliable and more updated than http://pool.sks-keyservers.net:11371.
Furthermore, they are resilient against the recent PGP attacks.

Checking the API specs of keys.openpgp.org it seems the change should be easy to do:

https://github.com/jkraemer/mail-gpg/blob/master/lib/hkp.rb#L86

What do you think of this change?
Thanks in advance.