MDL-65204 phpunit: update composer to use phpunit 7.5.x

To generate a new composer.lock for a given Moodle branch:
- Remove composer.lock
- Remove vendor
- Remove composer caches:
      php composer.phar clearcache
- Using lower PHP version supported (7.1.x for Moodle 3.7), update:
      php composer.phar update

https://docs.moodle.org/dev/Composer#How_to_prepare_and_submit_composer_changes

composer.json

@@ -5,8 +5,8 @@
     "type": "project",
     "homepage": "https://moodle.org",
     "require-dev": {
-        "phpunit/phpunit": "6.5.*",
-        "phpunit/dbunit": "3.0.*",
+        "phpunit/phpunit": "7.5.*",
+        "phpunit/dbunit": "4.0.*",
         "moodlehq/behat-extension": "3.37.0",
         "mikey179/vfsstream": "^1.6"
     }