Update `tag_filter_type` query parameter to be clearer

* Reports now use the "combination of" phrase to describe the union of
  tags
Bump ruby version (suggested in #14)

* While "fuzzy" ruby versions cannot be used because of rbenv, the
  repo's ruby version should be the highest patch level possible.
Add documentation about the "errors" array used for validation errors

* To provide clear and concise explanations for why an API request
  failed, an "errors" object is returned, which is an array of JSON
  objects that explain what resource and field had an error, and what
  the error message is. These error "codes" can be defined on a
  per-action level, and will be documented in the Resource's API page