Make pkg backwards compatible or fail well with older CKAN instance versions

[sckott]

via https://discuss.ropensci.org/t/new-package-ckanr/251/5

[sckott]

• adding new function ckan_version() to get the version for the server, then we can act accordingly
• may need to have an internal pkg environment to cache server version though b/c would suck to have to make the http request to get server version before every function call a user makes - then we can just look up the server version if it's already been called that session

[sckott]

things to catch:

• v2.5.1 - package_purge API action added
• v2.3 - New package_patch action
• v2.2
  • offset and limit params added to package_list action
  • The ckan.api_url configuration option has been completely removed and it can no longer be used
• v2.0
  • The Action API is now CKAN's default API, and the API documentation has been rewritten
• v - ...