Travis: add baseline stage

[blueyed]

Fixes #3876.

TODO:

• Can/should this be done with AppVeyor, too?
• keep py34 with normal/test stage
• rebase on Travis: report coverage with all builds #2800 to make sure nothing is missed

[nicoddemus]

Can/should this be done with AppVeyor, too?

AFAIK AppVeyor doesn't support stages.

keep py34 with normal/test stage

I don't mind much keeping it in the baseline.

rebase on #2800 to make sure nothing is missed

You want to do this before merging?

[nicoddemus]

Great work!

[coveralls]

Coverage increased (+0.05%) to 92.641% when pulling cdc72bf on blueyed:baseline into 9620b16 on pytest-dev:master.

[codecov]

Codecov Report

Merging #3894 into master will increase coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3894      +/-   ##
==========================================
+ Coverage   92.59%   92.64%   +0.05%     
==========================================
  Files          51       51              
  Lines        9960     9960              
==========================================
+ Hits         9222     9227       +5     
+ Misses        738      733       -5

Impacted Files	Coverage Δ
src/_pytest/terminal.py	94.42% <0%> (+0.92%)	⬆️

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[nicoddemus]

Awesome, thanks!