Update composer.json with the new moodlehq/behat-extension tag for 3.9dev

Requirements

terminal access and PHP 7.2 cli

In the master branch only:

Test 0

1. Skip this step, nothing to do, dear tester, integrators will deal with this. Verify that both phpunit and behat runs are installing and passing ok for master.

Test 1

(better use composer >= 1.3, to avoid some time format differences)

1. Open a terminal and navigate to your moodle root directory.
2. Run php composer.phar update
3. Open composer.lock on a text editor.
4. You SHOULD find
   "name": "moodlehq/behat-extension",
   "version": "v3.39.1",
   with "url": "https://github.com/moodlehq/moodle-behat-extension.git",
5. Back on your terminal, delete the "vendor" folder and "composer.lock". Also remove composer caches.

   rm -rf vendor

   rm composer.lock

   php composer.phar clearcache

6. Run again php composer.phar update
7. Open composer.lock on a text editor
8. You SHOULD find
   "name": "moodlehq/behat-extension",
   "version": "v3.39.1",
   with "url": "https://github.com/moodlehq/moodle-behat-extension.git",

Note: the local run of composer update may lead to some other small differences (git diff composer.lock) if some dependency packages were updated the last hours @ packagist. They can be ignored, unless there is something really big detected.