Set common deployment targets for the whole Xcode project
Set common deployment targets for the whole Xcode project

Setting the iOS deployment target at the project level ensures the IPHONEOS_DEPLOYMENT_TARGET environment variable is available to the scripts which build the external libraries.
Before this change, linking the external libraries with SwiftGit2-iOS resulted in hundreds of warnings that "object file (...) was built for newer iOS version (10.1) than being linked (9.2)".

SwiftGit2/SwiftGit2,mattrubin/SwiftGit2,SwiftGit2/SwiftGit2,mattrubin/SwiftGit2