Merge branch 'release/2.0.0'

* release/2.0.0:
  fix: handle case with no last attempted
  fix: fill dificulty nans
  major: split last_attempted_relevance into guess and slip args in recommendation_score
  docs: get_recommend_params note
  refactor: log formatting

v1.2.1 - Engine refactoring and test framework (harvard-vpal#13)

* Use relevance instead of guess and slip in function args

* feat: make BaseAdaptiveEngine more generic

* refactor: merge last attempted slip and guess into relevance param, rename mastery odds params for clarity

* Bump version

* feat: docker runtime and travis config

* Update gitignore

bump version in setup

Bump to 1.1.2 due to wrong pypi upload

Bump to 1.1.1

Bump version to 1.1.0

Bump version to 1.0.0

Bump version to 0.0.4