Install ruby before bundler

Author: alexcrichton

azure-pipelines.yml

@@ -14,6 +14,10 @@ jobs:
           curl -L https://github.com/rust-lang-nursery/mdBook/releases/download/v0.2.1/mdbook-v0.2.1-x86_64-unknown-linux-musl.tar.gz | tar xzf -
           echo "##vso[task.prependpath]$PWD"
         displayName: Install mdbook
+      - task: UseRubyVersion@0
+        inputs:
+          versionSpec: '>= 2.4'
+          addToPath: true
       - script: bundler install
         displayName: Install ruby deps
       - script: ./ci/build.sh