Fix requirement for standalone neo4j-core Gem

The gem built from the supplied gem spec would cause programs to fail with a:

```
neo4j-core-3.0.0.alpha.1/lib/neo4j/property_validator.
rb:9:in `<module:PropertyValidator>': uninitialized constant Neo4j::PropertyValidator::Set (NameError
)
```

The require within the PropertyValidator fixes this.