daru dependency as >= 0.2.0, change version to 0.1.1

[Yuki-Inoue]

In response to https://github.com/SciRuby/daru-io/issues/78#issuecomment-466979801 , making this PR.

In my second thought, I thought it would be better if we lower bound the dependency version, instead of using ~> constraint.

Reason is that since the release frequency of Daru and Daru-io seems to be different, using ~> constraint makes it cumbersome to manage the dependency in of this project. Instead, I thought it would be better if we simply just use the lower bound.

Any opinion?