fix: restore node 4 min support

* node 4 min
* add node 4 to travis

.gitignore

@@ -18,3 +18,5 @@ node_modules
 coverage
 tmp
 test/fixtures/test*
+
+.idea

.travis.yml

@@ -8,6 +8,7 @@ node_js:
   - '9'
   - '8'
   - '6'
+  - '4'
 before_install:
   - if [ "$TRAVIS_PULL_REQUEST_BRANCH" == "" ]; then echo "//registry.npmjs.org/:_authToken=\${NPM_TOKEN}" > .npmrc; fi
 after_success:

package.json

@@ -9,7 +9,7 @@
     "nodemon": "./bin/nodemon.js"
   },
   "engines": {
-    "node": ">=6"
+    "node": ">=4"
   },
   "repository": {
     "type": "git",