[#619] Fix broken dumps link on dataset page

An if statement in a template was wrong, so if dumps_url was _not_ set
then ckan would try to show the dumps link and would render an empty
string. Reversing the if statements fixes it.
Merge pull request #620 from okfn/619-bug-broken-dump-link-at-bottom-of-dataset-page

[#619] Fix broken dumps link on dataset page