Introduce CKAN versions to support CKAN API changes

[florianm]

The CKAN API has deprecated some endpoints which are still active in ckanr, e.g.
packages_activity_list works at demo.ckan.org but is replated in CKAN 2.10 by recently_changed_packages_activity_list.

Run against a local CKAN 2.10: localhost:5000/api/3/action/package_activity_list?id=ckanr_test_dataset returns "Bad request - Action name not known: package_activity_list"

From the CKAN 2.7 release notes:

The activity stream related actions ending with *_list (eg package_activity_list) and *_html (eg package_activity_list_html) will be removed in CKAN 2.8 in favour of more efficient alternatives and are now deprecated.

To support older and newer CKAN versions, ckanr could

• Add a ckanr_setup variable "CKAN_VERSION" with getters and setters
• Alternatively, retrieve/confirm the CKAN version somehow (can't find in CKAN docs) from the running CKAN
• Toggle functions depending on get_ckan_version() between working and warning ("This function has been deprecated/added in CKAN v2.x and is no longer / not yet available in the configured CKAN {CKAN_URL}, version {CKAN_VERSION}")