Adapted test matrix, fixes #36 and #32

[FinalAngel]

• Fixed test matrix
• Exclude tests folder from release build Do not install tests #36
• Added missing migrations for Django 2.1 Improve readability of Folder.get_files #32
• Improved readability of Folder.get_files

[codecov-io]

Codecov Report

Merging #37 into master will increase coverage by 1.35%.
The diff coverage is 57.14%.

@@            Coverage Diff             @@
##           master      #37      +/-   ##
==========================================
+ Coverage   83.04%   84.39%   +1.35%     
==========================================
  Files          12       13       +1     
  Lines         171      173       +2     
  Branches       11       10       -1     
==========================================
+ Hits          142      146       +4     
+ Misses         28       26       -2     
  Partials        1        1

Impacted Files	Coverage Δ
djangocms_file/models.py	72.85% <0%> (+2.02%)	⬆️
...angocms_file/migrations/0011_auto_20181211_0357.py	100% <100%> (ø)

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