Replacing the deprecated istanbul project with nyc for test coverage.

Author: arithmetric

.gitignore

@@ -1,2 +1,3 @@
+/.nyc_output
 /coverage
 /node_modules

package.json

@@ -4,9 +4,9 @@
   "description": "Serverless email forwarding using AWS Lambda and SES",
   "main": "index.js",
   "scripts": {
-    "check-coverage": "istanbul check-coverage --statements 100",
+    "check-coverage": "nyc --statements 100 npm run test",
     "lint": "eslint *.js example/*.js test/*.js",
-    "test": "istanbul cover _mocha -- --check-leaks --timeout 3000"
+    "test": "mocha -- --check-leaks --timeout 3000"
   },
   "dependencies": {
     "aws-sdk": "~2.631.0"
@@ -15,8 +15,8 @@
     "coveralls": "^3.0.7",
     "eslint": "^3.15.0",
     "eslint-config-google": "~0.6.0",
-    "istanbul": "^0.4.5",
-    "mocha": "^6.2.2"
+    "mocha": "^6.2.2",
+    "nyc": "^15.1.0"
   },
   "engines": {
     "node": ">=8.0"