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.
We can't cross compile for `hub` since it has native dependencies, e.g., [the x509 cert issue](https://groups.google.com/forum/?fromgroups#!searchin/golang-nuts/x509%2420failed%2420to%2420load%2420system%2420roots%2420and%2420no%2420roots%2420provided%7Csort:relevance/golang-nuts/tIK1ON3Pq6s/kCTpDht9vLUJ). What we could leverage `gox` for though is to cross compile for current OS, e.g., linux/386 and linux/amd64