Fix ClassCastException in AbstractVertexCentricQueryBuilder

Addresses https://github.com/thinkaurelius/titan/issues/651

Add a check for Conditions to ensure they can be casted to PredicateCondition
before attempting the cast while looking for equality constraints in the
AbstractVertexCentricQueryBuilder
Merge pull request #653 from mgmuscari/fix-query-builder

Fix ClassCastException in AbstractVertexCentricQueryBuilder