Fix for 'cljx once' is not a task. See 'lein help'.

Create a new project using the `+cljx` argument and then run the server with `lein ring server`. It fails with the error:

     $ lein ring server
    WARNING!!! version ranges found for:
    [com.keminglabs/cljx "0.4.0"] -> [org.clojars.trptcolin/sjacket "0.1.0.6"] -> [org.clojure/clojure "[1.3.0,)"]
    Consider using [com.keminglabs/cljx "0.4.0" :exclusions [org.clojure/clojure]].
    [com.keminglabs/cljx "0.4.0"] -> [org.clojars.trptcolin/sjacket "0.1.0.6"] -> [net.cgrand/regex "1.1.0"] -[org.clojure/clojure "[1.2.0,)"]
    Consider using [com.keminglabs/cljx "0.4.0" :exclusions [org.clojure/clojure]].
    [com.keminglabs/cljx "0.4.0"] -> [org.clojars.trptcolin/sjacket "0.1.0.6"] -> [net.cgrand/parsley "0.9.1"] -> [org.clojure/clojure "[1.2.0,)"]
    Consider using [com.keminglabs/cljx "0.4.0" :exclusions [org.clojure/clojure]].
    [com.keminglabs/cljx "0.4.0"] -> [org.clojars.trptcolin/sjacket "0.1.0.6"] -> [net.cgrand/parsley "0.9.1"] -> [net.cgrand/regex "1.1.0"] -> [org.clojure/clojure "[1.2.0,)"]
    Consider using [com.keminglabs/cljx "0.4.0" :exclusions [org.clojure/clojure]].

    'cljx once' is not a task. See 'lein help'.
 
The attached patch fixes this.