Your request is probably missing a parameter.
Foursquare has changed it's api as of January 28, 2014.
"Requests that do not include a v parameter will be rejected."
You will need to add a v
parameter which will be a date that represents the "version" of the API you expect from Foursquare.
v=YYYYMMDD
Clients that wish to be robust against changes should always pass v=DATEVERIFIED (in the YYYYMMDD format) and watch for any deprecated responses coming back from the server.
For more information visit:
https://developer.foursquare.com/overview/versioning