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 https://github.com/django-cms/djangocms-text-ckeditor/pull/650
• fix: Remove legacy code from Django pre 1.4 by @fsbraun in https://github.com/django-cms/djangocms-text-ckeditor/pull/651
• fix: Issue #641 created by typo in widgets.py by @fsbraun in https://github.com/django-cms/djangocms-text-ckeditor/pull/652
• Feature/issue 648 unable to unlink by @jrief in https://github.com/django-cms/djangocms-text-ckeditor/pull/649

[codecov[bot]]

Codecov Report

All modified lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/django-cms/djangocms-text-ckeditor/pull/653?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=django-cms) | Coverage Δ | | |---|---|---| | [djangocms\_text\_ckeditor/\_\_init\_\_.py](https://app.codecov.io/gh/django-cms/djangocms-text-ckeditor/pull/653?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=django-cms#diff-ZGphbmdvY21zX3RleHRfY2tlZGl0b3IvX19pbml0X18ucHk=) | `100.00% <100.00%> (ø)` | |

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