Use Yarn to manage dependencies (#576)

ashmaroli · web-flow · commit 7d6868c2b204 · 2019-12-30T12:56:15.000+05:30
Since we already check-in `yarn.lock` to the repository, it would be better if CI builds used the same set.
diff --git a/script/bootstrap b/script/bootstrap
@@ -4,4 +4,4 @@ set -e
 
 script/branding
 bundle install
-npm install
+yarn install
