chore: upgrade to newest version of istanbul codebase (#848)

BREAKING CHANGE: babel@7 drops Node.js 4 support

Author: bcoe

.travis.yml

@@ -3,7 +3,6 @@ language: node_js
 node_js:
   - 8
   - 6
-  - 4
   - "stable"
 git:
   depth:

appveyor.yml

@@ -3,7 +3,6 @@ environment:
     - nodejs_version: '10' # stable
     - nodejs_version: '8' # LTS
     - nodejs_version: '6'
-    - nodejs_version: '4'
 install:
   - ps: Install-Product node $env:nodejs_version
   - npm i npm@5 -g