This podspec was originally lifted from mapbox/mapbox-ios-sdk, which was structured and built quite differently. We could have the podspec build this repo as is, using scripts/package_ios.sh, but that would require developers to install the same version of Xcode as Travis is using (see #977). So this podspec instead pulls a prebuilt library. Much simpler.
Also package a LICENSE with the archive to satisfy a CocoaPods warning.
Fixes#829.
xcodebuild does not require a specific SDK version, but Mason puts packages in a version-specific directory. So this change uses the logic for $MASON_PLATFORM_VERSION in mason.sh in mapbox/mason.
Fixes#974.
