fix(tests): use devicemapper in builder functional test

After removing the VOLUME /var/lib/docker definition in the Dockerfile, the builder functional tests began failing:
```
2014/10/01 23:01:53 docker daemon: 1.2.0 fa7b24f; execdriver: native; graphdriver: aufs
[2d23ea72] +job serveapi(unix:///var/run/docker.sock)
2014/10/01 23:01:53 backing file system is unsupported for this graph driver
```
This works around docker's incompatible FS error by using devicemapper as the storage driver for the test.