Automatic tests for each supported React version

[f1ames]

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We have concluded with @Comandeer that it will be worth having integration tests which check if CKEditor 4 works correctly with all supported React versions - majors only I suppose. Generally, it should work be we don't check it too often. This will:

• Protect us from any backwards incompatibilities when introducing changes.
• Allow us quickly adding new tests whenever new React version is out.

It could be done probably in the form of simple application for each React version which will use our component (there are few ways to create such app - manual, CRA, etc. but we should get with the most common one). Also it should be fairly easy to add new tests for new versions.

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If it works fine we should do the same for other integrations.

[f1ames]

AFAIR the simplest idea proposed by @Comandeer was to just go with npm i react@version (or have predefined packages with relevant deps versions) and the run automatic tests for each version.

[CKEditorBot]

Closed in #166