Disable FOREIGN_KEY_CHECKS when purging database

[alexislefebvre]

When the parameter liip_test_fixtures.keep_database_and_schema: true is enabled, I have the following output:

SQLSTATE[23000]: Integrity constraint violation: 1451 Cannot delete or update a parent row: a foreign key constraint fails (truebadges.Badge, CONSTRAINT FK_3F316719F6BD1646 FOREIGN KEY (site_id) REFERENCES Site (id))

…/vendor/doctrine/dbal/lib/Doctrine/DBAL/Driver/AbstractMySQLDriver.php:49
…/vendor/doctrine/dbal/lib/Doctrine/DBAL/DBALException.php:169
…/vendor/doctrine/dbal/lib/Doctrine/DBAL/DBALException.php:145
…/vendor/doctrine/dbal/lib/Doctrine/DBAL/Connection.php:1063
…/doctrine/data-fixtures/lib/Doctrine/Common/DataFixtures> /Purger/ORMPurger.php:132
…/vendor/doctrine/data-fixtures/lib/Doctrine/Common/DataFixtures/Executor/AbstractExecutor.php:133
…/vendor/liip/test-fixtures-bundle/src/Services/DatabaseTools/ORMDatabaseTool.php:148
…/vendor/liip/test-fixtures-bundle/src/Test/FixturesTrait.php:87
…/tests/SEBundle/Command/CalculateSitesMinMaxCommandTest.php:33

This PR disables FOREIGN_KEY_CHECKS before purging the database.