Staging v1.6 release

[smmaurer]

This PR stages the v1.6 release, featuring new cache management functionality contributed by @bridwell (PR #46).

Changelog for v1.6

• New functionality to clear cache and update cache scope: https://udst.github.io/orca/core.html#caching
• Server module deprecated

We're deprecating the server module here, and plan to remove it in the subsequent release. It makes some users uncomfortable to have a web server automatically bundled with this package, and it also adds to the complexity of testing, distribution, and maintenance. I am not aware of anyone who still uses these components, but if it would be helpful we can reconsider or spin them off into a separate package.

This release still formally supports Python 2.7 and 3.5, but it's becoming difficult to run automated tests on these older platforms. The subsequent release will end support.

Maintenance and release prep included in this PR

• automated testing moved from Travis to GitHub Actions
• doc building instructions and scripts harmonized
• version number updated in init, setup, and docs
• entry added to changelog
• documentation updated to discuss new cache management functionality (already live at the link above)
• docstrings adjusted a bit from PR Cache clearing, update cache scope #46 for clarity and sphinx compatibility
• deprecation note added to documentation page for the server module
• copyright year updated in license and docs
• PyPI short description updated to match GitHub
• removed 'beta' status from the PyPI classifiers

[github-actions]

Test coverage is 97%