Replace exec-sync dep with node 0.12 child_process execSync

[deathcap]

The 'exec-sync' module has native dependencies, couldn't get it to easily build on my system (OS X), and https://www.npmjs.com/package/exec-sync 's repository https://github.com/jeremyfa/node-exec-sync says "IMPORTANT: This repository is no longer maintained. For the same feature, use this instead: https://github.com/mgutz/execSync " - but that module is also unmaintained, though it points out an alternative:

• both iojs and node.js v0.12 have execSync function (http://strongloop.com/strongblog/whats-new-in-node-js-v0-12-execsync-a-synchronous-api-for-child-processes/)

Replaced exec-sync in browserify-commit-sha with child_process execSync on node v0.12, works great in my testing. (This does remove browserify-commit-sha compatibility with older node versions like v0.10, if older compatibility is desired this (GPL) module looks like a decent choice: https://www.npmjs.com/package/sync-exec - didn't try it myself though)

[kumavis]

thanks for this by the way, much appreciated.