Travis: run CS in separate workflow & other minor tweaks

[jrfnl]

Context

• Improve stability of CI runs

Summary

This PR can be summarized in the following changelog entry:

• Improve stability of CI runs

Relevant technical choices:

As discussed in Slack this updates the Travis script to run the CS check in its own workflow.
Includes minor other tweaks and clean up.

For the record: this was the PHPCS PR which caused the difference in reported issues: squizlabs/PHP_CodeSniffer#2925

Composer: update lock file

.. to get rid of the The lock file is not up to date with the latest changes in composer.json. You may be getting outdated dependencies. warning.

Travis: remove cruft remaining after Premium split-off

Remove ENV variable declaration and conditions which are no longer necessary since the Premium plugin has been split off and now has its own script.

Travis: run CS checks in a separate build

... with its own simplified build steps.

Composer: update the CS threshold numbers

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

• Check out this branch
• Run composer install
• Run composer check-cs-thresholds and confirm that the threshold numbers are 100% correct.
• Check the Travis output - in particularly, the output of the separate PHP 7.4: check CS run.
  • Verify that the installed PHPCS version is 3.5.8.
  • And confirm again that the threshold numbers are 100% a match.

[enricobattocchi]

Everything's OK! merging