Use fetch for CKAN

When using Data Kitten with CSVlint, I noticed that some CKAN packages were not returning the correct metadata if the dataset name was generic (for example: http://data.gov.uk/dataset/index-of-multiple-deprivation, the first result was actually for the Welsh index of multiple deprivation). 

To fix this, I've used the CKAN `package_show` method for CKAN 3, and the `dataset` method for CKAN 2, which returns the actual dataset we want.