Pinned mocha version for compatibility with old IEs

package.json

@@ -15,7 +15,7 @@
   "main"            : "./index.js",
 
   "devDependencies" : {
-                        "mocha":     "*",
+                        "mocha":     "1.21.5",
                         "benchmark": "*",
                         "ansi":      "*",
                         "browserify":"*",