Drop 'date' property from gemspec

[stephenyeargin]

The gem.date property isn't necessary (defaults to the current date). Additionally, the date library is no longer included by default in Bundler, so this is causing some issues with updating things through @dependabot.

Reference: https://docs.ruby-lang.org/en/2.5.0/Gem/Specification.html Reference: https://github.com/rubygems/bundler/issues/5470

---

Dependabot can't evaluate your Ruby dependency files.

As a result, Dependabot couldn't check whether any of your dependencies are out-of-date.

The error Dependabot encountered was:

Bundler::GemspecError with message: [!] There was an error while loading gtfs.gemspec: uninitialized constant Date. Bundler cannot continue.

#  from /home/dependabot/dependabot-updater/vendor/ruby/2.6.0/bundler/gems/gtfs-a25bb6b9376b/gtfs.gemspec:8
#  -------------------------------------------
#    gem.version = GTFS::VERSION
   gem.date    = Date.today.to_s
#   
#  -------------------------------------------

[stephenyeargin]

Test fixes in #43