message: Fix mdbook version
diff: @@ -16,7 +16,7 @@ cache:
  before_script:
  # TODO: Installing mdbook from crates.io takes quite a long time. This could
  # be reduced by downloading the release from GitHub directly.
  - - mdbook -V || cargo install mdbook --vers '^0.3.0'
  + - mdbook -V || cargo install mdbook --vers '=0.3.1'
  script:
  - mdbook build
  - cp website_config.json book/
  