a small mistake in environment.yml

Dear authors,

There is a small mistake in Line 19 in [environment.yml](https://github.com/boschresearch/MetaBO/blob/master/environment.yml):

Missing one "=" in "scikit-learn=0.21.3", should be "scikit-learn==0.21.3"

User can only finish [Installation](https://github.com/boschresearch/MetaBO/tree/master#installation) after manually fixing.

I wish you can fix this.