The travis CI build for php 7.4 breaks due to phpunit failures.
Apparently, all of the failing checks have to do with serialization / de-serialization:
1) Wikibase\DataModel\Tests\ReferenceListTest::testGetValueHashIsTheSameForClone
2) Wikibase\DataModel\Tests\ReferenceListTest::testSerializationStability
3) Wikibase\DataModel\Tests\ReferenceListTest::testSerializeUnserializeRoundtrip
4) Wikibase\DataModel\Tests\ReferenceListTest::testUnserializeCreatesNonIdenticalClones
5) Wikibase\DataModel\Tests\Statement\StatementTest::testSerialize
In particular, assumptions such as:
```
$clone = unserialize( serialize( $original ) );
$this->assertTrue( $original->equals( $clone ) );
$this->assertSame( $original->getValueHash(), $clone->getValueHash() );
```
do not seem to be valid any longer, in PHP 7.4
Example failures: https://travis-ci.org/wmde/WikibaseDataModel/jobs/641447598