Update to Django / django CMS / latest highlightjs

[yakky]

General upgrade to 2019

• Bumping test dependencies
• Update highlightjs code
• Minor code changes for compatibility

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (develop@d18feaa). Click here to learn what that means.
The diff coverage is 100%.

@@           Coverage Diff            @@
##             develop     #8   +/-   ##
========================================
  Coverage           ?   100%           
========================================
  Files              ?      3           
  Lines              ?     40           
  Branches           ?      1           
========================================
  Hits               ?     40           
  Misses             ?      0           
  Partials           ?      0

Impacted Files	Coverage Δ
djangocms_highlightjs/settings.py	100% <ø> (ø)
djangocms_highlightjs/models.py	100% <100%> (ø)
djangocms_highlightjs/cms_plugins.py	100% <100%> (ø)

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[coveralls]

Coverage remained the same at ?% when pulling c6270df on feature/bump-versions into d18feaa on develop.