travis build env switched to xenial

[David Bremner]

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.

Those of you about to tell me how much better FreedomAvocadoCI [2] is
than Travis are encouraged to set up instances and we can try it out. As
long as Carl doesn't object, I would be interested in a setup where we
run e.g. webhooks directly on git.notmuchmail.org to remove our
dependence on github for CI.

d

[1]: this being the 20th anniversary of C99.
[2]: Not (yet) a real product/project.
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