Improve CI integration on all repos

[AliSoftware]

This is a global TODO list regarding improving CI integration for all repos. Each item could be later split in separate issues (but as we're still finishing up the Great Split I didn't want to pollute with too much issues just yet, but still have a TODO list to keep things in mind)

• Activate Danger, especially for
  • Checking the CHANGELOG entry has been added and it respects the expected format (full stop + 2 spaces)
  • Comment the PR with build warnings
• Migrate from Travis-CI to Circle-CI
• Separate each different steps (lint code, lint output, compile output, run unit tests…) in separate pipelines (that could be run in parallel) instead of having all the commands run one after the other in the same job. This way we can easily see which step worked and which failed
• Make sure each failing step does make the build fail. e.g. currently failure in linting doesn't make the CI job fail so the PR is still marked green even if linting had warnings and errors
• Make Danger push the lint warnings as a PR comment

\cc @djbe feel free to complete the list if you see anything missing

[djbe]

Maybe we can try to also parallelize the steps themselves, for example:

• build files in parallel (tests and output files)
• run tests in parallel?
• ...

[djbe]

From SwiftGen/SwiftGenKit#26:

• Use danger to add a warning if a submodule does not point to master
• Maybe something to trigger tests in related repos when needed

[djbe]

Saw this in slack:
https://circleci.com/docs/2.0/defining-multiple-jobs/

Would be a solution for triggering related builds using certain conditions.

[AliSoftware]

Interesting indeed. Will try to discuss that with Danielle this week but could be the right solution

[djbe]

Note: check readme links using danger
SwiftGen/SwiftGen#318 (comment)