CKAN Versions support going forward

We are planning on significantly extending and improving the current features and for that we need to focus our efforts. New features are python3 dependent, so users will need to choose the right ckanext-validation version depending on their CKAN version, with the goal that eventually everybody will be using CKAN 2.9+ on Python 3.

- [x] ckanext-validation 0.0.8 supports CKAN <= 2.8
- [x] ckanext-validation 1.0.0 will support CKAN 2.8 and 2.9 (py2 and py3). This is almost done in https://github.com/frictionlessdata/ckanext-validation/pull/55
- [ ] ckanext-validation 2.0.0 will support CKAN >= 2.9 with Python 3 support only. Here's where we will do the planned work, including replacing goodtables with frictionless-py, which is Python 3 only

Once #55 is done I'll release the new version and update the README with these guidelines

cc @aivuk @ThrawnCA

**edit**: added support for CKAN 2.8 in 1.0.0