Update mapSeries.js

[sempostma]

Here an array is being created with a set length:

https://github.com/benjamingr/bluebird-api/blob/1f02a97e30037943a2952449615fe6bf7ef28b39/promiseFns/mapSeries.js#L6

The concat function adds an item to array and increments length. Either ret[index] = value should be used or the length of the array should be initialized to 0.

https://github.com/benjamingr/bluebird-api/blob/1f02a97e30037943a2952449615fe6bf7ef28b39/promiseFns/mapSeries.js#L10