travis: switch from EOLed 'precise' to 'trusty'

[warner]

closes ticket:2888

[codecov-io]

Codecov Report

Merging #423 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #423      +/-   ##
==========================================
+ Coverage   88.02%   88.03%   +0.01%     
==========================================
  Files         147      147              
  Lines       27833    27833              
  Branches     3977     3977              
==========================================
+ Hits        24500    24503       +3     
+ Misses       2621     2618       -3     
  Partials      712      712

Impacted Files	Coverage Δ
src/allmydata/immutable/upload.py	95.25% <0%> (-0.28%)	⬇️
src/allmydata/mutable/servermap.py	94.04% <0%> (-0.17%)	⬇️
src/allmydata/mutable/publish.py	96.66% <0%> (+1.01%)	⬆️

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