Download and build Go 1.2 in the Dockerfile.

The dependency BurntSushi/toml actually needs Go 1.2, because it uses
encoding.TextUnmarshaler, which didn't exist in Go 1.1. Since the PPA
that we use doesn't have Go 1.2 yet, we will use the same method as
Docker, i.e. download Go source tarball and compile it.
Merge pull request #474 from jpetazzo/switch-to-go-1.2

Download and build Go 1.2 in the Dockerfile.