OPENEUROPA-2258: Use PHP 7.2 in drone and docker image.

[dxvargas]

OPENEUROPA-2258

Description

Use PHP 7.2 in drone and docker image.

Change log

• Added: PHP 7.2 usage in docker-compose and drone.

[dxvargas]

It was needed to add dev requirement "guzzlehttp/guzzle": "~6.3" to avoid errors in tests.
PhpUnit:

Drupal\Tests\oe_media_embed\Functional\MediaEmbedDialogTest::testMediaEmbedDialog
count(): Parameter must be an array or an object that implements Countable

Behat:

Warning: count(): Parameter must be an array or an object that implements Countable in vendor/guzzlehttp/guzzle/src/Handler/CurlFactory.php line 67