Add workflows for CircleCI so that `build_stable` and `build_bleeding_edge`...

Add workflows for CircleCI so that `build_stable` and `build_bleeding_edge` jobs can run concurrently.

.circleci/config.yml

@@ -45,3 +45,10 @@ jobs:
       - run:
           name: Execute test suite
           command: 'cd build ; ctest --output-on-failure -j 2'
+
+workflows:
+  version: 2
+  build_and_test_all:
+    jobs:
+      - build_stable
+      - build_bleeding_edge