Readme reflects min macOS requirement of 10.11

Summary: Closes #1089

Differential Revision: D6918813

Pulled By: rnystrom

fbshipit-source-id: 13586a4ce3515538f6ef8f8ccebd58900d1fc306

Author: Ryan Nystrom

README.md

@@ -49,7 +49,7 @@ We use the open source version `master` branch in the Instagram app.
 - Xcode 9.0+
 - iOS 8.0+
 - tvOS 9.0+
-- macOS 10.10+ *(diffing algorithm components only)*
+- macOS 10.11+ *(diffing algorithm components only)*
 - Interoperability with Swift 3.0+
 
 ## Installation