travis: use ppa:notmuch/notmuch

https://launchpad.net/~notmuch/+archive/ubuntu/notmuch/+packages
contains backports of gmime and gpgme and related cryptographic tools
to ubuntu xenial.

I tried to do a simple backport of gmime alone, and it failed due to
the older gpgme in xenial, so this setup resolves those problems.

This should allow us to continue to use the Travis continuous
integration build.

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>