travis build env switched to xenial

[David Bremner]

David Bremner <david at tethera.net> writes:

> I was feeling a bit annoyed at the idea of patching the notmuch test
> framework so that C99 was used by default [1], so I bumped the travis
> build env to xenial. trusty (the previous config) is EOL in April.
>
> Unfortunately it looks like xenial is also too old to have gmime-3.0, so
> we're still testing against libgmime-2.6.
>

On a related note, I've added the xapian backports ppa (maintained by
xapian upstream) to start testing against Xapian 1.4 by default. There
is some rough plan to remove support for pre-1.4 Xapian, so this is a
step in that direction. So far I didn't see an easy way to do something
similar to update the tested version of GMime

d