Remove gem.date from the gemspec.

This was causing errors when deploying to engine yard and heroku. I've seen many
other repos have this same issue. Another solution could be adding require
'date' to the gemspec, but I find setting gem.date to today's date somewhat
useless.
Merge pull request #77 from bbauer/remove_date_from_gemspec

Remove gem.date from gemspec
Merge remote-tracking branch 'upstream/master'
unit/filters/gist_test, update unit test