Try to use native process.execSync and fallback to polyfill if it's n…

…ot available.

Node.JS > 0.10 has it already.
Refernece: mgutz/execSync#20

lib/git.js

@@ -10,7 +10,7 @@
 'use strict';
 
 var logger = require('./logger'),
-    execSync = require('execSync'),
+    execSync = require('child_process').execSync || require('execSync'),
     spawn = require('child_process').spawn;
 
 exports.checkout = function(branch, opt_newBranch, opt_callback) {