v1.6.0

Fixed

• write_civis.numeric now correctly syncs with headers [#150].
• Use upper case for REST verb names. [#153].

Added

• write_civis gains header, credential_id and import_args arguments to more flexibly import data ([#150, #149, #130]).
• coef.civis_ml which returns civis_ml model coefficients in the format of stats::coef
  added tests for coef.civis_ml
• Added get_feature_importance to civis_ml_utils
• Added tests for get_feature_importance to test_civis_ml_utils
• Added feature_importances.rds to tests/testthat/data