Build universal framework target for Carthage

[1ec5]

MapboxDirections.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 CONTRIBUTING.md both need to be updated to refer to the new combined scheme.

/ref mapbox/turf-swift#197
/cc @mapbox/navigation-ios @OdNairy