message: Prepare changelog for v0.13.1
diff: +Changes in [0.13.1](https://github.com/matrix-org/matrix-react-sdk/releases/tag/v0.13.1) (2018-08-20)
  +=====================================================================================================
  +[Full Changelog](https://github.com/matrix-org/matrix-react-sdk/compare/v0.13.1-rc.1...v0.13.1)
  +
  + * No changes since rc.1
  +
  Changes in [0.13.1-rc.1](https://github.com/matrix-org/matrix-react-sdk/releases/tag/v0.13.1-rc.1) (2018-08-16)
  ===============================================================================================================
  [Full Changelog](https://github.com/matrix-org/matrix-react-sdk/compare/v0.13.0...v0.13.1-rc.1)
  