Revert "Enable strict mode and fix the TypeError"

This reverts commit 65e402e.

.eslintrc

@@ -3,8 +3,5 @@
   "env": {
     "node": true,
     "es6": true
-  },
-  "rules": {
-    "strict": "error"
   }
 }

index.js

@@ -1,5 +1,3 @@
-'use strict';
-
 const parser = require('@babel/parser');
 
 /**
@@ -79,10 +77,7 @@ module.exports.prototype.traverse = function(node, cb) {
       // Avoid visited nodes
       if (key === 'parent' || !node[key]) { continue; }
 
-      if (typeof node[key] === 'object') {
-        node[key].parent = node;
-      }
-
+      node[key].parent = node;
       this.traverse(node[key], cb);
     }
   }

test/test.js

@@ -1,7 +1,5 @@
 /* eslint-env mocha */
 
-'use strict';
-
 const assert = require('assert');
 const Walker = require('../');
 const fs = require('fs');