Fix Travis CI

[tony]

Fixes #304

Also upgrades Travis CI build from precise -> trusty

[codecov-io]

Codecov Report

Merging #348 into master will decrease coverage by 6.82%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #348      +/-   ##
==========================================
- Coverage   82.49%   75.67%   -6.83%     
==========================================
  Files           5        5              
  Lines         777      777              
  Branches        0      226     +226     
==========================================
- Hits          641      588      -53     
  Misses        136      136              
- Partials        0       53      +53

Impacted Files	Coverage Δ
tmuxp/config.py	85.36% <0%> (-8.95%)	⬇️
tmuxp/cli.py	57.05% <0%> (-7.06%)	⬇️
tmuxp/util.py	90.9% <0%> (-6.07%)	⬇️
tmuxp/workspacebuilder.py	90.9% <0%> (-4.25%)	⬇️

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