package: use relative file path for "needs-ie-fallback"

For some reason, browserify works without a relative
path when being invoked from within the `get-window` directory,
but fails when outside of it.

Now fixed.

Author: TooTallNate

package.json

@@ -15,7 +15,7 @@
   ],
   "main": "index.js",
   "browser": {
-    "./needs-ie-fallback.js": "needs-ie-fallback.br.js",
+    "./needs-ie-fallback.js": "./needs-ie-fallback.br.js",
     "jsdom": false
   },
   "dependencies": {