add missing var for antiMeridian variable

[fknop]

Should close #32

[dereekb]

fwiw you might consider changing it to be:

    allowAntiMeridian = allowAntiMeridian || false;

    var cacheKey = tileSize + String(allowAntiMeridian);
    if (!smCache[cacheKey]) {
        smCache[cacheKey] = new SphericalMercator({ size: tileSize, antimeridian: allowAntiMeridian });
    }

That way it is in line with how they used tileSize and are setting a default value to the input variable.