Added certificate file to podspec resources (as per @mmorey suggestion)

Corrects issue when Forecast.io calls fail in apps using Forecastr as a
pod due to the forecast.io.cer not being copied into the app bundle.

Problem manifests itself with the following error during Forecast.io
service calls: "Error Domain=NSURLErrorDomain Code=-999 "cancelled"

Fix provided by @mmorey.