racket/gui cocoa: fix for Mac OS X before 10.7

I forgot to guard a NSScreen method called for Retina support
with a version check.

pkgs/gui-pkgs/gui-lib/mred/private/wx/cocoa/dc.rkt

@@ -110,6 +110,7 @@
    (tellv ctx restoreGraphicsState)))
 
 (define (display-bitmap-resolution num fail)
+  (if (version-10.7-or-later?)
       (let ([r (atomically
                 (with-autorelease
                  (let ([s (if (zero? num)
@@ -123,7 +124,8 @@
         (cond
          [(not r) (fail)]
          [(zero? r) 1.0]
-     [else r])))
+         [else r]))
+      1.0))
 
 (define/top (make-screen-bitmap [exact-positive-integer? w]
                                 [exact-positive-integer? h])