jmespath.js parser error showing undefined instead of actual token type and value

When the jmespath parser handles a syntax error related to parsing slice expressions, it's throwing an error message:
```
Syntax error: unexpected token: undefined(undefined)
```
Which, it should actually have token and value instead of `undefined`.

In the Parser's `_parseSliceExpression()` function, it looks like this:
```
var t = this._lookahead(0);
var error = new Error("Syntax error, unexpected token: " +
t.value + "(" + t.type + ")");
```

Solution: it should use `this._lookaheadToken()` instead of `this._lookahead()`. The this._lookaheadToken() will return a token object that will actually contain .type and .value properties