Use -n auto now that xdist behaves well in Travis and AppVeyor

[nicoddemus]

This hopefully fixes the flaky test_idval_hypothesis on AppVeyor

Fix #3707

[nicoddemus]

Ok nothing makes sense anymore. 😁

https://travis-ci.org/pytest-dev/pytest/jobs/424539670 is executing with 48 workers, even though we have code in place to avoid that...

[coveralls]

Coverage increased (+0.02%) to 94.081% when pulling 8be29c1 on nicoddemus:idval-hypothesis-flaky-3707 into 410d576 on pytest-dev:master.

[codecov]

Codecov Report

Merging #3933 into master will increase coverage by 0.53%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3933      +/-   ##
==========================================
+ Coverage   94.46%   94.99%   +0.53%     
==========================================
  Files         108      108              
  Lines       23432    23436       +4     
  Branches     2329     2332       +3     
==========================================
+ Hits        22136    22264     +128     
+ Misses        989      881     -108     
+ Partials      307      291      -16

Flag	Coverage Δ
#doctesting	28.68% <ø> (ø)	⬆️
#linux	?
#nobyte	90.03% <ø> (+90.03%)	⬆️
#numpy	28.32% <ø> (+0.02%)	⬆️
#pexpect	47.83% <ø> (?)
#pluggymaster	93.74% <ø> (+0.71%)	⬆️
#py27	93.04% <ø> (+1.01%)	⬆️
#py34	92.28% <ø> (+0.15%)	⬆️
#py35	92.29% <ø> (+0.74%)	⬆️
#py36	93.29% <ø> (+1.17%)	⬆️
#py37	92.46% <ø> (+0.73%)	⬆️
#trial	31.25% <ø> (-0.14%)	⬇️
#windows	?
#xdist	93.23% <ø> (+74.76%)	⬆️

Impacted Files	Coverage Δ
testing/python/metafunc.py	95.15% <ø> (-0.01%)	⬇️
testing/test_tmpdir.py	95.06% <0%> (-4.94%)	⬇️
testing/code/test_code.py	93.98% <0%> (-1.37%)	⬇️
src/_pytest/nodes.py	95.77% <0%> (-0.94%)	⬇️
src/_pytest/pytester.py	85.71% <0%> (-0.31%)	⬇️
testing/test_capture.py	98.94% <0%> (-0.31%)	⬇️
src/_pytest/fixtures.py	96.99% <0%> (-0.28%)	⬇️
testing/code/test_source.py	96.8% <0%> (ø)	⬆️
testing/acceptance_test.py	98.01% <0%> (+0.22%)	⬆️
src/_pytest/capture.py	90.16% <0%> (+0.22%)	⬆️
... and 12 more

---

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