Ammends SearchStocks Type

[ArianZargaran]

Context

search Endpoint returns a Promise but declared as RawStockSearch instead. According to

What was done?

According to search tests, used as an async call, it's clear the Endpoint returns. a Promise.
https://github.com/zackurben/alphavantage/blob/master/test/data.js#L94

[coveralls]

Coverage decreased (-77.0%) to 16.97% when pulling 41c29e1 on ArianZargaran:ammend-searchstocks-type into d7cb8d4 on zackurben:master.

[zackurben]

Thanks for the contribution, sorry for the delay!