fix node4 build issue (dequelabs#542)

* fix(node4): use var to declare variables

Block-scoped declarations (let, const, function, class) not yet supported outside strict mode

* feat(package): add engine hint to package.json

provides minimal requirements warning

build/utils/postinstall.js

@@ -1,9 +1,9 @@
 var fs = require('fs')
 
 if (fs.existsSync(".git/hooks")) {
-	let commitMsg = '.git/hooks/commit-msg'
+	var commitMsg = '.git/hooks/commit-msg'
 	if (!fs.existsSync(commitMsg)) {
-		let angularPrecommit = "./node_modules/angular-precommit/index.js"
+		var angularPrecommit = "./node_modules/angular-precommit/index.js"
 		if (fs.existsSync(angularPrecommit)) {
 			var fsExtra = require('fs-extra')
 			fsExtra.copy(angularPrecommit, commitMsg, (err) => {
@@ -12,4 +12,4 @@ if (fs.existsSync(".git/hooks")) {
 			})
 		}
 	}
-}
+}

package.json

@@ -3,6 +3,9 @@
   "description": "Accessibility engine for automated Web UI testing",
   "version": "3.0.0-alpha.4",
   "license": "MPL-2.0",
+  "engines": {
+    "node": ">=4"
+  },
   "contributors": [
     {
       "name": "David Sturley",