Add step to save Redis URLs from Heroku

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The Heroku Redis URLs for the PR/beta/canary deployments change
periodically, which breaks things until I manually update the Redis URL
environment variables for the Travis environment. This obtains them
at build time from Heroku instead.

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This also moves travis-web jobs onto the Trusty beta, because it includes the heroku command needed to fetch the Redis URLs. This has the added benefit of including PhantomJS 2.0.0, which comes pre-installed in that environment, so we no longer need the .travis.yml lines related to installing that.