add tests, passing along maintainer role

[Yue-Jiang]

• Add tests for non-plotting functions for randomForest and ranger forests
• Add .travis.yml
• Moving over the maintainer role to me (also realized roxygen2 doesn't allow multiple maintainers, so would like to check with you guys)

[codecov-io]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@32ca649). Click here to learn what that means. The diff coverage is 60%.

@@            Coverage Diff            @@
##             master      #11   +/-   ##
=========================================
  Coverage          ?   56.43%           
=========================================
  Files             ?        4           
  Lines             ?      629           
  Branches          ?        0           
=========================================
  Hits              ?      355           
  Misses            ?      274           
  Partials          ?        0

Impacted Files	Coverage Δ
R/explain_forest.R	0% <0%> (ø)
R/min_depth_interactions.R	56.87% <61.53%> (ø)
R/measure_importance.R	61.67% <80%> (ø)

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