Pass TRAVIS env var to test environments [skip appveyor]

[nicoddemus]

xdist has a workaround inplace for Travis so "-n auto" works.

Fix #4162

[nicoddemus]

py27-nobyte:

gw0 [2263] / gw1 [2263]

😎

[codecov]

Codecov Report

Merging #4163 into master will decrease coverage by 0.14%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4163      +/-   ##
==========================================
- Coverage    95.8%   95.66%   -0.15%     
==========================================
  Files         109      109              
  Lines       23828    23828              
  Branches     2352     2352              
==========================================
- Hits        22829    22794      -35     
- Misses        708      735      +27     
- Partials      291      299       +8

Flag	Coverage Δ
#docs	?
#doctesting	?
#linting	?
#linux	95.66% <ø> (ø)	⬆️
#nobyte	91.28% <ø> (-0.63%)	⬇️
#numpy	41.42% <ø> (-51.58%)	⬇️
#pexpect	41.42% <ø> (ø)	⬆️
#py27	93.9% <ø> (-0.11%)	⬇️
#py34	92.23% <ø> (-0.09%)	⬇️
#py35	92.25% <ø> (-0.09%)	⬇️
#py36	93.89% <ø> (-0.17%)	⬇️
#py37	92.41% <ø> (+0.03%)	⬆️
#trial	41.42% <ø> (-51.58%)	⬇️
#windows	?
#xdist	93.65% <ø> (-0.14%)	⬇️

Impacted Files	Coverage Δ
testing/test_paths.py	86.36% <0%> (-13.64%)	⬇️
src/_pytest/paths.py	91.3% <0%> (-8.7%)	⬇️
testing/test_tmpdir.py	95.06% <0%> (-4.94%)	⬇️
src/_pytest/capture.py	90.16% <0%> (-3.21%)	⬇️
src/_pytest/nodes.py	93.69% <0%> (-0.85%)	⬇️
testing/acceptance_test.py	97.59% <0%> (-0.66%)	⬇️
src/_pytest/pytester.py	86.47% <0%> (-0.46%)	⬇️
testing/test_capture.py	98.94% <0%> (-0.31%)	⬇️
src/_pytest/fixtures.py	97.02% <0%> (-0.28%)	⬇️

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