Reduce PropEr test size for speed

I need to fix this so that object sizes don't explode when generating
larger values. Basically, as the type generator recurses is should be
adjusting the size value.

test/jiffy_tests.erl

@@ -10,7 +10,7 @@
 proper_test_() ->
     PropErOpts = [
         {to_file, user},
-        {max_size, 30},
+        {max_size, 15},
         {numtests, 1000}
     ],
     {timeout, 3600, ?_assertEqual([], proper:module(jiffy_tests, PropErOpts))}.