GPG keys update is needed for 1.6 and 1.7 APT repositories

[Alexander Turenko]

For users of APT repositories 1.6 and 1.7: the next metadata update will show the error (on 1.6 and 1.7 correspondingly):

> The following signatures couldn't be verified because the public key is not available: NO_PUBKEY DCF5FADF3C1D16C1
> The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 20C3A9DB75CC7EDE

It is because of packagecloud gpg keys migration, see [1]. The fix is the following (for 1.6 and 1.7):

> curl http://download.tarantool.org/tarantool/1.6/gpgkey | sudo apt-key add -
> curl http://download.tarantool.org/tarantool/1.7/gpgkey | sudo apt-key add -

More information is available in the relevant issue: [2].

[1]: https://packagecloud.io/docs#gpg_migration

[2]: https://github.com/tarantool/tarantool/issues/3736

WBR, Alexander Turenko.