Fix tests on CI services

[Chilipp]

• Tests added (for all bug fixes or enhancements)
• Tests passed (for all non-documentation changes)
• Fully documented, including CHANGELOG.rst for all changes

This PR fixes several CI issues that we had in the past. In particular

• import skimage.feature at conftest.py (0be07de)
• added xvfb services on travis (dbe505c)
• use Miniconda36 instead of Miniconda37 on Appveyor (223c629)
• only test the beginner tutorial on OSX on Travis (030e7d4)
• tells psyplot-gui that we are running tests (cc284c7)
• installs libgl1-mesa-glx and libegl1-mesa-dev on travis for the QtWebEngineWidgets (183703a)

[codecov]

Codecov Report

Merging #10 into master will increase coverage by 0.2%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master     #10     +/-   ##
========================================
+ Coverage    85.5%   85.7%   +0.2%     
========================================
  Files          27      27             
  Lines       11739   11739             
========================================
+ Hits        10037   10061     +24     
+ Misses       1702    1678     -24

Impacted Files	Coverage Δ
straditize/straditizer.py	89% <0%> (+0.1%)	⬆️
straditize/widgets/tutorial/beginner.py	94.38% <0%> (+0.28%)	⬆️
straditize/widgets/tutorial/hoya_del_castillo.py	83.55% <0%> (+0.33%)	⬆️
straditize/cross_mark.py	86.27% <0%> (+0.37%)	⬆️
straditize/widgets/colnames.py	91.96% <0%> (+3.8%)	⬆️

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