Allow buck to be downloaded and called with multiple targets.

It turns out that the way that we constructed the lambda
containing the buck invocation meant that we'd always be
including the first target passed to it, no matter how
many other invocations there were. *sigh* One day we'll
all know how to write nice Ruby. :)

rake-tasks/buck.rb

@@ -44,11 +44,13 @@ def self.download
   end
 
   def self.buck_cmd
-    @@buck_cmd ||= (
+    (
       lambda { |command, args, &block|
-        args ||= []
-        buck = Buck::download
+        buck = []
+        pex = Buck::download
+        buck.push(*pex)
 
+        args ||= []
         buck.push(command)
         buck.push(*args)
 
@@ -89,7 +91,6 @@ def output
       }
     )
 
-    #block.call(output) if block
   end
 
   def self.enhance_task(task)