Clean up package script

* Prefer Ruby script to `gotask` script for packaging task
* Clean up Vagrant file
* Prefer `gox` to `goxc`

To build for Darwin, assuming you're running the task on OSX:

```
$ script/package
```

To build for Linux:

```
$ vagrant up
$ vagrant ssh
$ cd ~/gocode/src/github.com/github/hub
$ script/package
```

The packaged files will be available in the target folder. Pending Vagrant settings for Windows.
It's changed in the later version of Vagrant to point to [Vagrant cloud](https://vagrantcloud.com/boxes). Here's the setting: https://docs.vagrantup.com/v2/vagrantfile/machine_settings.html.