Rework Travis config

[ryanhiebert]

• Use Travis stages to put deployment in its own Job
• Use Xenial dist (16.04), requires using apt for rabbitmq. sudo=False not supported.
• Add Python 3.7 testing.

[codecov[bot]]

Codecov Report

Merging #53 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #53   +/-   ##
=======================================
  Coverage   54.63%   54.63%           
=======================================
  Files          11       11           
  Lines         410      410           
=======================================
  Hits          224      224           
  Misses        186      186

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