Improve how we include scientist

We shipped a fix in scientist 1.6.0 that minimizes our issues around changing the default class, and helped fix a bug in regards to when it's included within a module.

We work around our concern here by simply moving the include into a module and pushing a nice little fix upstream as well. Yay open source process!

Note: This doesn't surface as a problem within our library exactly, it has more to do with the behavior of Scientist::Experiment.new instantiating Scientist::Default or Gitlab::Experiment.

/cc @dreedy