WIP Use xenial build image on Travis [skip appveyor]

[nicoddemus]

Now we can use Python 3.7 without sudo

https://blog.travis-ci.com/2018-11-08-xenial-release

[codecov]

Codecov Report

Merging #4364 into master will decrease coverage by 0.2%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4364      +/-   ##
==========================================
- Coverage   95.83%   95.63%   -0.21%     
==========================================
  Files         111      111              
  Lines       24907    24907              
  Branches     2431     2431              
==========================================
- Hits        23869    23819      -50     
- Misses        738      775      +37     
- Partials      300      313      +13

Flag	Coverage Δ
#docs	?
#doctesting	?
#linting	?
#linux	95.63% <ø> (+0.01%)	⬆️
#nobyte	91.22% <ø> (-0.64%)	⬇️
#numpy	41.6% <ø> (-51.4%)	⬇️
#pexpect	41.6% <ø> (ø)	⬆️
#py27	93.91% <ø> (-0.1%)	⬇️
#py34	92.03% <ø> (-0.07%)	⬇️
#py35	92.04% <ø> (-0.07%)	⬇️
#py36	93.74% <ø> (-0.15%)	⬇️
#py37	92.21% <ø> (+0.06%)	⬆️
#trial	41.6% <ø> (-51.4%)	⬇️
#windows	?
#xdist	93.57% <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	90.24% <0%> (-3.63%)	⬇️
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%)	⬇️
... and 1 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e14ca19...870c6ad. Read the comment docs.

[blueyed]

Followup: #4365