feat: Bump to 5.1.4

[fsbraun]

This PR prepares the release of version 5.1.4:

• fix: test suite for Django 2.2 by @fsbraun in fix: test suite for Django 2.2 #650
• fix: Remove legacy code from Django pre 1.4 by @fsbraun in fix: Remove legacy code from Django pre 1.4 #651
• fix: Issue The old plugin content is deleted after clicking cancel with new nested plugins #641 created by typo in widgets.py by @fsbraun in fix: Issue #641 created by typo in widgets.py #652
• Feature/issue 648 unable to unlink by @jrief in Feature/issue 648 unable to unlink #649

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (4e5c4bd) 76.33% compared to head (0d4821b) 76.33%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #653   +/-   ##
=======================================
  Coverage   76.33%   76.33%           
=======================================
  Files          20       20           
  Lines         917      917           
  Branches      123      123           
=======================================
  Hits          700      700           
  Misses        191      191           
  Partials       26       26           

Files	Coverage Δ
djangocms_text_ckeditor/__init__.py	100.00% <100.00%> (ø)

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