Update list of release signing keys
The download page says that All the new releases are signed with either Dmitry's, Tim's, or Daiki's OpenPGP key.
However:
- 3.7.4 is signed with 7A75A648B3F9220C, which appears to be Zoltan's key; this isn't on the list.
- The link to Dmitry's key is broken (although it's available from other keyservers).