Handle NCDFE for odd envs like GAE that throw it for Unsafe.

Fixes #2304.

Author: headius

core/src/main/java/org/jruby/util/unsafe/UnsafeHolder.java

@@ -50,6 +50,9 @@ private static sun.misc.Unsafe loadUnsafe() {
             return (sun.misc.Unsafe) f.get(null);
         } catch (Exception e) {
             return null;
+        } catch (NoClassDefFoundError ncdfe) {
+            // Google AppEngine raises NCDFE for Unsafe rather than CNFE
+            return null;
         }
     }