Fix unittest failures on 32bit machines (test date too large)

email.utils.parsedate() uses mktime, which eventually
uses time_t, which can only represent up to the
32bit unix epoch (aka, 03:14:08 UTC on 19 January 2038)

Change future date in test to date less than that.
Merge pull request #55 from candlepin/alikins/too_far_back_in_future

Fix unittest failures on 32bit machines (test date too large)