TST: sometimes failing ujson comparison
cc  @Komnomnomnom

skipping test via: b5ff81d34d74efdf88b8151ff58ed16ff7c02c67

fails about 1/3 times (and only seems on py3)

https://travis-ci.org/pydata/pandas/jobs/10223886

don't use now, pick a known value (that you can then compare too exactly)

```
  def test_datetime_units(self):
        from pandas.lib import Timestamp

        val = datetime.datetime.now()
        stamp = Timestamp(val)
```
