Adjust deployment target of macOS

[Marcus Wu]

Hi @yonaskolb , currently the deployment target of macOS in generated spec is hard code to s.osx.deployment_target = '10.9', maybe we need update it for newly Alamofire version.

Possible solution:

1. Add options to set the deployment target version
2. Adjust the template hard code value to 10.10

Which one is prefer? I can create a PR for it. 😀

[Akira MATSUDA]

This issue may be solved in 4.5.0 :)