update installation instructions with new repo
update installation instructions with new repo

change mentions of https://riot.im/packages to https://packages.riot.im
add instructions to remove old, now-untrusted riot.im signing key
update language and move from 'apt-get` to the simpler `apt`
vector-im/vector-web,vector-im/riot-web,vector-im/vector-web,vector-im/vector-web,vector-im/vector-web,vector-im/riot-web,vector-im/riot-web,vector-im/riot-web,vector-im/vector-web,vector-im/riot-web,vector-im/riot-web