Fix test_cron_string_to_datetime_from_time for OSX

OSX supports times after about 1902, while other UNIX-based distros
support 1900 onward. I suggest more recent dates are used in tests
for compatibility.
Merge pull request #140 from rackspace-titan/mktime_out_of_range

Fix test_cron_string_to_datetime_from_time for OSX