Include git tag SHA in build

Use the `-ldflags` to change version at the linker. See http://golang.org/cmd/ld. I also deleted `script/godep` since it causes problem when I pass the `go build` command to the script - bash trims all quotes when passing arguments to `script/godep`, failing the build.

Issue: https://github.com/github/hub/issues/672#issuecomment-62040388

```
$ ./script/build
$ ./hub version
git version 2.0.1
hub version 2.2.0-preview1-37-g9ea3492
```

/cc @mislav