Minor refactoring of the .circleci config.yaml (#1885)

* Adding in "<<: *defaults" under "coverage"

* Refactoring by capturing similar setup commands into single alias

* Reverting changes, I am not certain exactly how to refactor these setup steps, but at least the &default refactor can be made.

.circleci/config.yml

@@ -53,8 +53,7 @@ jobs:
           name: Unit tests
           command: npm run test
   coverage:
-    docker:
-      - image: circleci/node:8
+    <<: *defaults
     steps:
       - checkout
       - restore_cache:
@@ -80,3 +79,4 @@ workflows:
       - coverage:
           requires:
             - dependencies
+