Slack changed their GPG key to https://slack.com/gpg/slack_pubkey_201…

…9.gpg per

this KB article:

https://slack.com/help/articles/115004809166-verify-slack-for-linux-(beta)-package-signatures

This PR updates the slack-repo to reflect that change.

slack.repo

@@ -3,7 +3,7 @@ name=Slack
 baseurl=https://packagecloud.io/slacktechnologies/slack/fedora/21/$basearch
 gpgcheck=1
 enabled=1
-gpgkey=https://slack.com/gpg/slack_pubkey.gpg
+gpgkey=https://slack.com/gpg/slack_pubkey_2019.gpg
 sslverify=1
 sslcacert=/etc/pki/tls/certs/ca-bundle.crt
 metadata_expire=300