Install and enable the mongodb extension for 7.1

[stronk7]

In this branch we had to, also, install the extension. Was not present.

Just that, nothing else needs to be done. moolde-docker will have
other changes to make the test environment ready.

This is part of #16

Unit tests are passing ok with 37_STABLE and master:

$ bin/moodle-docker-compose exec webserver vendor/bin/phpunit --testsuite cachestore_mongodb_testsuite -v
Moodle 3.7+ (Build: 20190524), e39f3b7db215b61d68ea2a23ff850f0efa1853e8
Php: 7.1.29, pgsql: 9.6.7, OS: Linux 4.9.125-linuxkit x86_64
PHPUnit 7.5.9 by Sebastian Bergmann and contributors.

Runtime:       PHP 7.1.29
Configuration: /var/www/html/phpunit.xml

..                                                                  2 / 2 (100%)

Time: 4.78 seconds, Memory: 52.00 MB

OK (2 tests, 42 assertions

And being skipped with previous Moodle versions (where MDL-54592) was not implemented:

$ bin/moodle-docker-compose exec webserver vendor/bin/phpunit --testsuite cachestore_mongodb_testsuite -v
Moodle 3.6.4+ (Build: 20190524), baebbcc341e44dc5552d54a2be8c6fdd3cf4d9c8
Php: 7.3.5, pgsql: 9.6.7, OS: Linux 4.9.125-linuxkit x86_64
PHPUnit 6.5.8 by Sebastian Bergmann and contributors.

Runtime:       PHP 7.3.5
Configuration: /var/www/html/phpunit.xml

SS                                                                  2 / 2 (100%)

Time: 4.45 seconds, Memory: 48.00MB

There were 2 skipped tests:

1) cachestore_mongodb_test::test_collection_name
Could not test cachestore_mongodb. Requirements are not met.

/var/www/html/cache/tests/fixtures/stores.php:52
/var/www/html/lib/phpunit/classes/advanced_testcase.php:80

To re-run:
 vendor/bin/phpunit -v "cachestore_mongodb_test" cache/stores/mongodb/tests/mongodb_test.php

2) cachestore_mongodb_test::test_test_instance
Could not test cachestore_mongodb. Requirements are not met.

/var/www/html/cache/tests/fixtures/stores.php:52
/var/www/html/lib/phpunit/classes/advanced_testcase.php:80

To re-run:
 vendor/bin/phpunit -v "cachestore_mongodb_test" cache/stores/mongodb/tests/mongodb_test.php

OK, but incomplete, skipped, or risky tests!
Tests: 2, Assertions: 0, Skipped: 2.