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:

```
$ 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.
Why only build for one platform? Shouldn't we let gox compile it for all the platforms we want to support?