fix intermittent build failures
fix intermittent build failures

Travis are currently doing a phased rollout of 14.04
for builds without an explicit `dist`. See
https://blog.travis-ci.com/2017-07-11-trusty-as-default-linux-is-coming

Build was passing if it ran on 12.04
and failing if it ran on 14.04
This pins the build to 14.04 and applies fixes
so it should now pass consistently.

DemocracyClub/EveryElection,DemocracyClub/EveryElection,DemocracyClub/EveryElection