TST: fix ujson's intermittently failing date unit test

[Komnomnomnom]

Fixes #4575.

As you surmised @jreback it looks like unnecessary float arithmetic was causing the test failure. I fixed this up and in the interest of consistency removed the dependence on datetime.now()

Example

In [8]: val = 1376774232215504000

In [9]: val / 1000
Out[9]: 1376774232215504

In [10]: val / 1e3
Out[10]: 1376774232215503.8

In [11]: val // 1e3
Out[11]: 1376774232215503.0

In [12]: val / 1000.0
Out[12]: 1376774232215503.8

In [13]: val / 10**3
Out[13]: 1376774232215504

[jreback]

merge?

[Komnomnomnom]

Yep, good to go. Passes for me on Python 2 & 3

[jreback]

thanks you sir!