chore: replace istanbul with nyc

Author: developher-net

.gitignore

@@ -19,6 +19,7 @@ coverage
 
 # nyc test coverage
 .nyc_output
+coverage.lcov
 
 # Grunt intermediate storage (http://gruntjs.com/creating-plugins#storing-task-files)
 .grunt

.travis.yml

@@ -2,10 +2,9 @@ language: node_js
 node_js:
   - "stable"
 install:
-  - npm install -g codecov istanbul
+  - npm install -g codecov
 script:
-  - istanbul cover ./node_modules/mocha/bin/_mocha --reporter lcovonly -- -R spec
-  - codecovn
+  - codecov
 jobs:
   include:
     # Define the release stage that runs semantic-release