Add runtime dependency on daru-io

Currently, `daru` is a runtime dependency of `daru-io` and `daru-io` is just added as a development dependency of `daru` in PR #430 to remove the infinite loop in bundle installing. Before `daru-1.0-pre` is released, `daru-io-0.1.1` has to be released and this change has to be made.

I just wanted to note this down as a to-do, in the v-1-pre milestone.