travis: explicitly specify dist

This makes the builds more reproducible as travis is currently rolling
out trusty as default dist [1]. Specifically, this avoids coverage
check failures when trusty is used as seen in [2] until we figure out
what's wrong.

[1] https://blog.travis-ci.com/2017-07-11-trusty-as-default-linux-is-coming
[2] https://github.com/curl/curl/pull/1692

Closes https://github.com/curl/curl/pull/1725