[Swift 2.3] update aerogear-ios-http + cookbook demo

Update from Swift 2.1 to Swift 2.3 syntax.
Use Xcode8 migration tools.
Update README for minimal requirement Xcode 7.3.
Deal with Xcode7 / Xcode 8 compatibility, see https://docs.google.com/document/d/1nFX6eGE_l5luklIjLOnCwD4jGjEqxEUYOWjwaVJxyNg/edit#heading=h.fknrjuvhu2xs

Cookbook demo:
https://github.com/aerogear/aerogear-ios-cookbook/tree/master/Weather