Redundant storage of operations

[melniknow]

The operations are stored in a List (expressions) and when the operation is overwritten, the old version remains in the Collection.

public final class JsonLogic {
  private final List<JsonLogicExpression> expressions;
  private final Map<String, JsonLogicNode> parseCache;
  private JsonLogicEvaluator evaluator;

  public JsonLogic() {
    this.expressions = new ArrayList<>();

[melniknow]

Fixed here: #40