Update travis CI jobs to use trusty

[kpaulisse]

Reference: https://blog.travis-ci.com/2017-07-11-trusty-as-default-linux-is-coming

Presently at least 2 of our 5 builds have trouble running under trusty:

https://travis-ci.org/github/octocatalog-diff/builds/259510369

Generally complaining, when things shell out, about something like:

/home/travis/.rvm/rubies/ruby-2.2.7/lib/ruby/site_ruby/2.2.0/rubygems.rb:270:in `find_spec_for_exe': can't find gem bundler (>= 0.a) (Gem::GemNotFoundException)