fix: stop bundling istanbul-lib-instrument due to npm issue on Node 6 (#854)

Author: bcoe

appveyor.yml

@@ -5,7 +5,6 @@ environment:
     - nodejs_version: '6'
 install:
   - ps: Install-Product node $env:nodejs_version
-  - npm i npm@5 -g
   - set PATH=%APPDATA%\npm;%PATH%
   - set CI=true
   - npm config set progress=false

package.json

@@ -85,6 +85,7 @@
     "glob": "^7.0.6",
     "istanbul-lib-coverage": "^1.2.0",
     "istanbul-lib-hook": "^1.1.0",
+    "istanbul-lib-instrument": "^2.1.0",
     "istanbul-lib-report": "^1.1.3",
     "istanbul-lib-source-maps": "^1.2.5",
     "istanbul-reports": "^1.4.1",
@@ -139,7 +140,6 @@
     "glob",
     "istanbul-lib-coverage",
     "istanbul-lib-hook",
-    "istanbul-lib-instrument",
     "istanbul-lib-report",
     "istanbul-lib-source-maps",
     "istanbul-reports",