Make build_book_master run in Linux

This was failing because it was running on windows: https://dev.azure.com/graphql-rust/GraphQL%20Rust/_build/results?buildId=223

Author: LegNeato

azure-pipelines.yml

@@ -28,6 +28,8 @@ jobs:
 
 - job: build_book_master
   displayName: Build rendered book on master branch and push to Github
+  pool:
+    vmImage: ubuntu-16.04
   dependsOn: run_book_tests
   condition: and(succeeded(), eq(variables['Build.SourceBranch'], 'refs/heads/master'))
   variables: