Changed dependency declaration to work w/ lein 2 + clojure 1.5.1

Hi Nathan,

I had problems resolving the dependency in your branch's README when qualified as
'cascalog/cascalog-core' (for 2.0.0-SNAPSHOT), but none with using 'cascalog,' which 
is also consistent with instructions here: 

https://clojars.org/cascalog/versions/2.0.0-SNAPSHOT .

Cheers, Russell