Require Redcarpet 2.3.0 and update Gemfile.lock.

Previously the curly quotes for apostrophes had to be worked around, but
this is no longer necessary as of 2.3.0, so the workaround is removed,
and relevant test code updated.
Better tests for different timezones.

Previously failing in timezones like -0400. Should be more
timezone-independent.
More resilient testing.

The trailing newline shouldn't be important here, so it's removed.