message: prepare travis configuration to support bors
diff: language: rust
  +branches:
  + only:
  + # This is where pull requests from "bors r+" are built.
  + - staging
  + # This is where pull requests from "bors try" are built.
  + - trying
  + # Uncomment this to enable building pull requests.
  + - master
  rust:
  - nightly
  os: linux
  