Update keyserver to fetch PHP GPG keys

Summary:
The SKS keyservers service is no longer provided,
as they can't be compliant with GDPR.

Test Plan:

gpg --keyserver keyserver.ubuntu.com --recv-keys \
        5A52880781F755608BF815FC910DEB46F53EA312 \
        42670A7FE4D0441C8E4632349E4FDC074A4EF02D \

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D2422