Planning Orca v1.6 release

[smmaurer]

This issue is to plan a v1.6 release of Orca.

Main updates

• cache management functionality added by @bridwell in PR Cache clearing, update cache scope #46.

Maintenance

• move automated tests to GitHub Actions
• update doc build process to match other UDST packages
• update versioning
• update README and docs
• resolve issue Broadcast erroneously claims cast_on and onto_on are equivalents of left_on and right_on parameters of pandas.merge. #37, at least by fixing the documentation
• update usage of pd.Series.to_csv(path=) for compatibility with Pandas 0.24+ (travis failure, docs)

Anything else we should include?