fa2056af1c travis: Properly cache and error on timeout (MarcoFalke)
fa36a333ee travis: Switch to ubuntu keyserver to avoid timeouts (MarcoFalke)
Pull request description:
The other keyserver is consistently timing out on travis: https://travis-ci.org/bitcoin/bitcoin/jobs/512689710#L405
Attempt to fix it by using a different server.
Also:
* fixes#15372
* fixes#15738
ACKs for commit fa2056:
ryanofsky:
utACK fa2056af1c71aded3a821a07ec4de71c4be0bca3. All good changes (changing keyserver, getting rid of keyserver while loop, clarifying travis error, moving travis documentation to code comment).
Tree-SHA512: ac8436616ecfee0ed579114e19f03c53ceb688fbcd95a60cffe8f15b4e569772a6ba673f353bbd789e79fe27fc5626c77fab4086768844dd51e0c6c108b52fb2
