Avoid deprecated fs.existsSync.

tools/install.js

@@ -1,10 +1,14 @@
 var fs = require('fs-extra');
 
-if (fs.existsSync('install.json')) {
+try {
   var installMap = fs.readJsonSync('install.json');
   for (src in installMap) {
     for (dest of installMap[src]) {
       fs.copySync(src, dest);
     }
   }
+} catch(err) {
+  if (err.code !== 'ENOENT') {
+    throw err;
+  }
 }