Fix BooleanHandler for PostgreSQL

This succeeds with MySQL and SQLite but fails with PostgreSQL. See https://travis-ci.org/wmde/WikibaseQueryEngine/jobs/42077613. Yes, this is probably a bug in DBAL but if there is a simple workaround, why not use it? I'm sure using ints instead of booleans (note that `false` becomes an empty string in PHP) is the most compatible way.

Wikibase\QueryEngine\QueryEngineException: An exception occurred while executing '`INSERT INTO qr_mainsnak_boolean (subject_id, subject_type, property_id, statement_rank, value) VALUES (?, ?, ?, ?, ?)`' with params `["Q20", "item", "P7701", 1, false]`:

SQLSTATE[22P02]: Invalid text representation: 7 ERROR:  invalid input syntax for type boolean: ""