fix: repair Travis builds

[Midnighter]

• reduce dependencies
• drop Python 3.4
• make use of new CPLEX

[Midnighter]

I had to include all changes from #207 in order to make this work.

[codecov-io]

Codecov Report

Merging #208 into devel will increase coverage by 0.15%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##            devel     #208      +/-   ##
==========================================
+ Coverage   63.84%   63.99%   +0.15%     
==========================================
  Files          56       56              
  Lines        5108     5108              
  Branches      882      882              
==========================================
+ Hits         3261     3269       +8     
+ Misses       1663     1655       -8     
  Partials      184      184

Impacted Files	Coverage Δ
cameo/models/webmodels.py	62.87% <0%> (+6.06%)	⬆️

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[Midnighter]

Just a note: I set up the installation of CPLEX only for the master branch and tagged commits so I couldn't test that yet. I hope it will work, though.