doc(README): Use ubuntu keyserver for apt-key
doc(README): Use ubuntu keyserver for apt-key

As pgp.mit.edu has become extremely unreliable, this switches to
keyserver.ubuntu.com for retrieval of package keys

Change-Type: patch

resin-io/resin-etcher,resin-io/etcher,resin-io/resin-etcher,resin-io/etcher,resin-io/etcher,resin-io/herostratus,resin-io/etcher,resin-io/herostratus,resin-io/etcher