Merge pull request #137 from tatsuya6502/mdbook-0.4.5

mdBook 0.4.5へアップグレード

Author: tatsuya6502

.circleci/config.yml

@@ -3,7 +3,7 @@ version: 2
 jobs:
   build:
     docker:
-      - image: quay.io/rust-lang-ja/circleci:mdbook-0.3.6-rust-1.41.0
+      - image: quay.io/rust-lang-ja/circleci:mdbook-0.4.5-rust-1.49.0
     parallelism: 1
     steps:
       - checkout