Bump Kotlin version to 1.2.50 stable (#386)
Bump Kotlin version to 1.2.50 stable (#386)

Summary:
* Bumps Kotlin version to 1.2.50 stable.

`sample-kotlin` builds successfully on that version.
Closes https://github.com/facebook/litho/pull/386

Differential Revision: D8466596

Pulled By: passy

fbshipit-source-id: d6095fac93b9e16886ff8dff2ffec5c79129f40a

facebook/litho,facebook/litho,facebook/litho,facebook/litho,facebook/litho,facebook/litho