Removed link to (nonexistent?) Meetup group

The link is dead, no relevant group seems to exist on the Stockholm (at least there are no search results for "Julia" within 100 miles of Stockholm...) and [this post on the Julia Users mailing list](https://groups.google.com/forum/?fromgroups=#!topic/julia-users/V6aLwuqMPaI) doesn't seem to trigger any response =(