Allow all SlugField characters in /data-sets urls

`\w` matches underscores but not hyphens. `[\w-]` matches both.

See http://stackoverflow.com/a/5718037 and http://regex101.com/r/uN6pP0

See https://www.pivotaltracker.com/story/show/66670412
[Fixes #66670412]
Merge pull request #77 from alphagov/unbreak-data-sets-urls

Allow all SlugField characters in /data-sets urls