Kernel version checking is broken

Running on Debian 7 with Kernel 3.8 trunk, Docker fails to start:

```
vagrant@vagrant-debian-wheezy-64:~$ uname -r
3.8-trunk-amd64
vagrant@vagrant-debian-wheezy-64:~$ uname -a
Linux vagrant-debian-wheezy-64 3.8-trunk-amd64 #1 SMP Debian 3.8.5-1~experimental.1 x86_64 GNU/Linux
vagrant@vagrant-debian-wheezy-64:~$ sudo ./docker -d
2013/04/22 03:08:10 Unrecognized kernel version
```
