fix TreeArbitrary. enable Tree traverse1 test

previous version generates too large Tree.



before

```scala
scala> def treeSize = scalaz.scalacheck.ScalazArbitrary.TreeArbitrary[Int].arbitrary.sample.get.flatten.size
treeSize: Int

scala> Array.fill(30)(treeSize)
res0: Array[Int] = Array(774951, 1485780, 190755, 198900, 1148848, 1262450, 16468, 1227201, 1589027, 944048, 928012, 1017635, 687690, 1117685, 563697, 1182292, 165075, 858252, 29559, 1173886, 1445796, 1169567, 999443, 712694, 1907499, 75679, 670961, 175509, 1680301, 1325979)
```


after

```scala
scala> def treeSize = scalaz.scalacheck.ScalazArbitrary.TreeArbitrary[Int].arbitrary.sample.get.flatten.size
treeSize: Int

scala> Array.fill(30)(treeSize)
res0: Array[Int] = Array(31987, 16720, 28175, 32919, 6853, 850, 33739, 4310, 14320, 12408, 9346, 2346, 1420, 7085, 11318, 29391, 7269, 29312, 40494, 3390, 24958, 38326, 26529, 13535, 40632, 18979, 9308, 42805, 1, 28803)
```