Some small fixes related to the documentation and the building process (Fix #842)

[unoebauer]

This small PR addresses a number of small issues which came up when resolving #872 (should also fix #842)

• add warning about need for Python 2.7
• update link to tardis_example.yml file on the docu
• add cython back into the env file

[unoebauer]

@wkerzendorf - did I insert the correct cython version? Also, are there some other small changes which we could/should include in this PR?

[unoebauer]

Ok, Travis obviously thinks that I have added the wrong cython version...

[wkerzendorf]

@unoebauer you did insert the "correct" cython version but I'm it still crashes with that. I think we need to handle this in the travis setup with an uninstall of cython and then reinstall using the self-built option.

[unoebauer]

ok, I'll give it a try

[codecov-io]

Codecov Report

Merging #873 into master will increase coverage by 1.46%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #873      +/-   ##
==========================================
+ Coverage   82.26%   83.72%   +1.46%     
==========================================
  Files          34       34              
  Lines        2870     2870              
==========================================
+ Hits         2361     2403      +42     
+ Misses        509      467      -42

Impacted Files	Coverage Δ
tardis/plasma/base.py	88.07% <0%> (+27.81%)	⬆️

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[unoebauer]

@unoebauer - done! Travis is happy now.

[wkerzendorf]

I almost fixed it. but then I saw that you did.