Update CI config

[Comandeer]

What is the purpose of this pull request?

Task

Does your PR contain necessary tests?

All patches that change the editor code must include tests. You can always read more on PR testing, how to set the testing environment and how to create tests in the official CKEditor documentation.

This PR contains

• [ ] Unit tests
• [ ] Manual tests

Did you follow the CKEditor 4 code style guide?

Your code should follow the guidelines from the CKEditor 4 code style guide which helps keep the entire codebase consistent.

• [x] PR is consistent with the code style guide

What is the proposed changelog entry for this pull request?

N/A

What changes did you make?

I've removed the Travis CI config and replaced it with the Circle CI one. The main change is the fact that now all runs are full runs (the diffing is disabled).

Which issues does your PR resolve?

N/A

[Comandeer]

@KarolDawidziuk, thx for the review.

• Updated the Docker image.
• Removed badge and replaced it with CircleCI's one.
• Removed Travis-oriented scripts.
• Removed config.isTravis variable from Bender's config and reenabled tests.
• Updated CKE4 build config to ignore .circleci dir instead of the .travis.yml file.