Add istanbul package for code coverage reports

Author: AndrewJo

circle.yml

@@ -9,6 +9,10 @@ checkout:
     - git config user.email "support@verdigris.co"
     - git config user.name "Verdigris Technologies"
 
+dependencies:
+  pre:
+    - npm install -g istanbul
+
 test:
   override:
     - istanbul cover ./tests/runner.js --report lcovonly && codecov