Build universal framework target for Carthage

[1ec5]

MapboxGeocoder.xcodeproj currently has four separate framework targets, one for each platform, plus corresponding test targets and schemes. To streamline development both locally and downstream, we should combine them into a single framework target and test target. Downstream, only Carthage uses the Xcode project; it has supported universal frameworks since v0.9. The CircleCI configuration and the readme both need to be updated to refer to the new combined scheme.

/ref mapbox/turf-swift#197 mapbox/mapbox-directions-swift#730