ci: Fix CircleCI error applying branch filtering at the workflow level

This commit attempts to fix the following error:

  Job "build" has filters configured in the job definition. These filters are incompatible with workflows.

See Slicer/Slicer#4906 (comment)

Author: jcfr

Diff for: .circleci/config.yml

@@ -4,9 +4,6 @@ jobs:
     working_directory: /usr/src/apidocs
     docker:
       - image: python:2.7.13
-    branches:
-    only:
-      - master
     steps:
       - checkout
       - run:
@@ -102,3 +99,12 @@ jobs:
               --slicer-repo-tag "${SLICER_REPO_TAG}" \
               --status-update-target-url "https://slicer.github.com/${PUBLISH_GITHUB_PROJECT_NAME}"
 
+workflows:
+  version: 2
+  build:
+    jobs:
+      - build:
+          filters:
+            branches:
+              only: master
+