Update composer dependencies and stabilize build

[JavierCane]

• Run composer update in order to update the composer.lock since the previous version didn't match with the composer.json one.
• Fix for deprecated YAML usages of parameters without being quoted
• Fix for "Invalid type for path "monolog.handlers.main.channels.elements.0". Expected scalar, but got object."
• Update JMS Serializer bundle in order to fix "ServiceNotFoundException: The service "jms_serializer.stopwatch_subscriber" has a dependency on a non-existent service "debug.stopwatch"". More info: Move symfony/stopwatch to require instead of require-dev schmittjoh/JMSSerializerBundle#624
• Fix for DateTimeSimilarComparator#assertEquals and DateTimeStringSimilarComparator#assertEquals not being compatible with SebastianBergmann\Comparator\ObjectComparator#assertEquals
• Fix failing vendor/bin/behat -p applications test due to invalid YAML
• Update Mockery in order to fix error due to PHP 7.2 deprecating count over nulls: "count(): Parameter must be an array or an object that implements Countable"
• Update PhpUnit to 6.5 in order to be compatible with Mockery 1.0
  • Make the UnitTestCase class extend from MockeryTestCase in order to close and check Mockery assertions. This is the recommended way since Mockery 1.0: http://docs.mockery.io/en/latest/reference/phpunit_integration.html
• Fix PhpUnit assertions usage from ApiResponseContext

[JavierCane]

With this PR we return to the greenzone of successful CircleCI builds 🙂