Travis: use Xenial by default [skip appveyor]

[blueyed]

No description provided.

[codecov]

Codecov Report

Merging #4365 into master will decrease coverage by 0.21%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4365      +/-   ##
==========================================
- Coverage   95.85%   95.64%   -0.22%     
==========================================
  Files         111      111              
  Lines       24910    24910              
  Branches     2431     2431              
==========================================
- Hits        23877    23824      -53     
- Misses        736      774      +38     
- Partials      297      312      +15

Flag	Coverage Δ
#docs	?
#doctesting	?
#linting	?
#linux	95.64% <ø> (ø)	⬆️
#nobyte	91.22% <ø> (-0.64%)	⬇️
#numpy	41.62% <ø> (-51.39%)	⬇️
#pexpect	41.62% <ø> (ø)	⬆️
#py27	93.9% <ø> (-0.13%)	⬇️
#py34	92.03% <ø> (-0.07%)	⬇️
#py35	92.04% <ø> (-0.07%)	⬇️
#py36	93.75% <ø> (-0.15%)	⬇️
#py37	92.21% <ø> (+0.06%)	⬆️
#trial	41.62% <ø> (-51.39%)	⬇️
#windows	?
#xdist	93.58% <ø> (-0.16%)	⬇️

Impacted Files	Coverage Δ
testing/test_pathlib.py	91.17% <0%> (-8.83%)	⬇️
src/_pytest/assertion/util.py	92.92% <0%> (-5.19%)	⬇️
testing/test_tmpdir.py	94.33% <0%> (-4.41%)	⬇️
src/_pytest/capture.py	90.7% <0%> (-3.18%)	⬇️
src/_pytest/pathlib.py	88.46% <0%> (-2.2%)	⬇️
testing/acceptance_test.py	97.2% <0%> (-1.08%)	⬇️
src/_pytest/nodes.py	93.95% <0%> (-0.81%)	⬇️
src/_pytest/pytester.py	86.96% <0%> (-0.43%)	⬇️
testing/test_capture.py	98.92% <0%> (-0.31%)	⬇️
src/_pytest/fixtures.py	97.13% <0%> (-0.27%)	⬇️

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[blueyed]

@nicoddemus
Please merge if it is OK.

[nicoddemus]

Awesome, thanks @blueyed!