Add a swift_versions flag to the Podspec.

[MrMage]

This squelches a warning in the Podspec before releasing the 0.9.1 tag.

The following warnings will still remain in the Podspec:

    - WARN  | xcodebuild:  /Users/daniel/Library/Developer/Xcode/DerivedData/App-ekzxujpqpbynmreaqbbxjewuznea/Build/Products/Release-iphonesimulator/gRPC-Core/grpc.framework/Headers/grpc_security.h:635:74: warning: this function declaration is not a prototype [-Wstrict-prototypes]
    - WARN  | xcodebuild:  /Users/daniel/Library/Developer/Xcode/DerivedData/App-ekzxujpqpbynmreaqbbxjewuznea/Build/Products/Release-iphonesimulator/gRPC-Core/grpc.framework/Headers/grpc_security.h:677:76: warning: this function declaration is not a prototype [-Wstrict-prototypes]
    - WARN  | [OSX] xcodebuild:  /Users/daniel/Library/Developer/Xcode/DerivedData/App-ekzxujpqpbynmreaqbbxjewuznea/Build/Products/Release/gRPC-Core/grpc.framework/Headers/grpc_security.h:635:74: warning: this function declaration is not a prototype [-Wstrict-prototypes]
    - WARN  | [OSX] xcodebuild:  /Users/daniel/Library/Developer/Xcode/DerivedData/App-ekzxujpqpbynmreaqbbxjewuznea/Build/Products/Release/gRPC-Core/grpc.framework/Headers/grpc_security.h:677:76: warning: this function declaration is not a prototype [-Wstrict-prototypes]
    - WARN  | [tvOS] xcodebuild:  /Users/daniel/Library/Developer/Xcode/DerivedData/App-ekzxujpqpbynmreaqbbxjewuznea/Build/Products/Release-appletvsimulator/gRPC-Core/grpc.framework/Headers/grpc_security.h:635:74: warning: this function declaration is not a prototype [-Wstrict-prototypes]
    - WARN  | [tvOS] xcodebuild:  /Users/daniel/Library/Developer/Xcode/DerivedData/App-ekzxujpqpbynmreaqbbxjewuznea/Build/Products/Release-appletvsimulator/gRPC-Core/grpc.framework/Headers/grpc_security.h:677:76: warning: this function declaration is not a prototype [-Wstrict-prototypes]

These are fixed in #499, but as the Podspec directly pulls in its own copy of gRPC-Core, the CocoaPods variant does not benefit from that fix. Unfortunately, I do not have the time to upgrade this whole project to gRPC-Core 1.22 right now, however.