Replace file-exists.js with NPM path-exists.

src/utils/file-exists.js

@@ -1,19 +0,0 @@
-/**
-Definition of the `fileExists` method.
-@license MIT. Copyright (c) 2015 James Devlin / FluentDesk.
-@module file-exists.js
-*/
-
-var FS = require('fs');
-
-// Yup, this is now the recommended way to check for file existence on Node.
-// fs.exists is deprecated and the recommended fs.statSync/lstatSync throws
-// exceptions on non-existent paths :)
-module.exports = function (path) {
-  try {
-    FS.statSync( path );
-    return true;
-  } catch( err ) {
-    return !(err && err.code === 'ENOENT');
-  }
-};