Use bash to run libuv install script

Running `make deps` on Ubuntu 13.10 causes the following errors

```
scripts/get-libuv.sh: 3: [: Linux: unexpected operator
scripts/get-libuv.sh: 5: [: Linux: unexpected operator
scripts/get-libuv.sh: 10: [: unknown: unexpected operator
```

I'm not sure if changing the runner to `bash` is the best way, but it fixed it for me :)