Fix tests on CI services

[Chilipp]

• [x] Tests added (for all bug fixes or enhancements)
• [x] 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 (https://github.com/Chilipp/straditize/commit/0be07de7cd7051764467269537a75b4be07f0888)
• added xvfb services on travis (https://github.com/Chilipp/straditize/commit/dbe505c5a3a9010441d250f9a969979c1aad9945)
• use Miniconda36 instead of Miniconda37 on Appveyor (https://github.com/Chilipp/straditize/commit/223c629d40309d45f76e85384c77921eca5ad87a)
• only test the beginner tutorial on OSX on Travis (https://github.com/Chilipp/straditize/commit/030e7d4a4805b0c7e2b67dde2da895b1f2e4f879)
• tells psyplot-gui that we are running tests (https://github.com/Chilipp/straditize/commit/cc284c7bb4de353d293e5a998b99c036013188af)
• installs libgl1-mesa-glx and libegl1-mesa-dev on travis for the QtWebEngineWidgets (https://github.com/Chilipp/straditize/commit/183703a3b4b002e04f8f1f77a3e9541c56067c4a)

[codecov[bot]]

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%)	:arrow_up:
straditize/widgets/tutorial/beginner.py	94.38% <0%> (+0.28%)	:arrow_up:
straditize/widgets/tutorial/hoya_del_castillo.py	83.55% <0%> (+0.33%)	:arrow_up:
straditize/cross_mark.py	86.27% <0%> (+0.37%)	:arrow_up:
straditize/widgets/colnames.py	91.96% <0%> (+3.8%)	:arrow_up:

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