Attempt to restore Node v4 support

Author: mde

.travis.yml

@@ -1,6 +1,7 @@
 language: node_js
 sudo: false
 node_js:
+  - "4"
   - "6"
   - "8"
   - "10"

postinstall.js

@@ -1,5 +1,7 @@
 #!/usr/bin/env node
 
+'use strict';
+
 function isTrue(value) {
   return !!value && value !== '0' && value !== 'false';
 }