Replaced istanbul with nyc for code coverage

Author: ryu1kn

.gitignore

@@ -1,5 +1,7 @@
-/node_modules
 /.idea
+/.nycrc
+/.nyc_output
 /.vscode
 /coverage
+/node_modules
 __*

.npmignore

@@ -4,6 +4,8 @@
 /.eslintrc.yaml
 /.gitignore
 /.idea
+/.nycrc
+/.nyc_output
 /.travis.yml
 /.vscode
 /coverage

package.json

@@ -5,7 +5,7 @@
   "main": "index.js",
   "scripts": {
     "test": "mocha test --recursive",
-    "coverage": "istanbul cover ./node_modules/.bin/_mocha -- test --recursive",
+    "coverage": "nyc mocha test --recursive",
     "lint": "eslint ."
   },
   "repository": {
@@ -28,8 +28,8 @@
     "codeclimate-test-reporter": "^0.5.0",
     "eslint": "^4.18.1",
     "eslint-config-xo": "^0.20.1",
-    "istanbul": "^0.4.5",
     "mocha": "^5.0.1",
+    "nyc": "^11.4.1",
     "sinon": "^4.4.2"
   }
 }