build(aio): move gulp tasks to package.json

Generate the docs with `yarn docs`.
Test the doc generation code with `yarn docs-test`

The docs are automatically built as part of the `yarn build` task,
so there is no need to rebuild them in the test_aio.sh file

scripts/ci/test-docs.sh

@@ -10,6 +10,6 @@ source ${thisDir}/_travis-fold.sh
 travisFoldStart "test.docs"
   (
     cd ${PROJECT_ROOT}/aio
-    $(npm bin)/gulp docs-test
+    yarn docs-test
   )
 travisFoldEnd "test.docs"