message: Bump mdbook version for contrib guide.
diff: @@ -14,7 +14,7 @@ jobs:
  - name: Install mdbook
  run: |
  mkdir mdbook
  - curl -Lf https://github.com/rust-lang/mdBook/releases/download/v0.4.5/mdbook-v0.4.5-x86_64-unknown-linux-gnu.tar.gz | tar -xz --directory=./mdbook
  + curl -Lf https://github.com/rust-lang/mdBook/releases/download/v0.4.9/mdbook-v0.4.9-x86_64-unknown-linux-gnu.tar.gz | tar -xz --directory=./mdbook
  echo `pwd`/mdbook >> $GITHUB_PATH
  - name: Deploy docs
  run: |
  