Build universal framework target for Carthage

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

首先检查 MapboxGeocoder.xcodeproj 及其现有的平台 framework target 和 test target，然后检查 CircleCI 配置和 README 中对 scheme 的引用。完成的标准是项目使用一个合并的 framework target 和 test target，并且 CI 配置和 README 已更新为引用合并后的 scheme。