Updated macOS deployment target to 10.9.

Mirroring the same change in AppAuth openid/AppAuth-iOS@8db5df5

GTMAppAuth.podspec

@@ -17,7 +17,7 @@ requests with AppAuth.
   #       obtaining authorization from the user. You can use the classes of
   #       GTMAppAuth with tokens obtained out of band to authorize requests
   #       on tvOS.
-  s.platforms    = { :ios => "7.0", :osx => "10.8", :tvos => "9.0" }
+  s.platforms    = { :ios => "7.0", :osx => "10.9", :tvos => "9.0" }
 
   s.homepage     = "https://github.com/google/GTMAppAuth"
   s.license      = "Apache License, Version 2.0"
@@ -37,7 +37,7 @@ requests with AppAuth.
 
   s.osx.source_files = "Source/GTMOAuth2KeychainCompatibility/*.{h,m}",
                        "Source/macOS/**/*.{h,m}"
-  s.osx.deployment_target = '10.8'
+  s.osx.deployment_target = '10.9'
 
   s.tvos.source_files = "Source/iOS/GTMKeychain_iOS.m"
   s.tvos.deployment_target = '9.0'