TST: test_encodeDoubleTinyExponential breaking on 32-bit
related #4299
cc @Komnomnomnom

weird that this doesn't break on travis (but did on my vagrant machine), and on debian testing....

any thoughts?

```
> /home/vagrant/pandas_master/pandas/io/tests/test_json/test_ujson.py(109)test_encodeDoubleTinyExponential()
-> self.assertEqual(num, ujson.decode(ujson.encode(num)))
(Pdb) l
104             num = 1e-100
105             self.assertEqual(num, ujson.decode(ujson.encode(num)))
106             num = -1e-45
107             self.assertEqual(num, ujson.decode(ujson.encode(num)))
108             num = -1e-145
109  ->         self.assertEqual(num, ujson.decode(ujson.encode(num)))
110  
111         def test_encodeDictWithUnicodeKeys(self):
112             input = { u"key1": u"value1", u"key1": u"value1", u"key1": u"value1", u"key1": u"value1", u"key1": u"value1", u"key1": u"value1" }
113             output = ujson.encode(input)
114  
(Pdb) p num
-1e-145
(Pdb) ujson.decode(ujson.encode(num))
-1.0000000000000001e-145
```
