specify rubyzip ~> 1.1

to reflect changes in https://github.com/nerdEd/gtfs/pull/18
gem now requires Ruby >= 1.9.2

Because that's what rubyzip ~> 1.1 requires.

And, for what it's worth, Ruby 1.8.7 and 1.9.2 are no longer supported.
version number 0.2.4
Merge pull request #21 from nerdEd/prepare-0.2.4

Prepare 0.2.4
Merge branch 'master' of https://github.com/nerdEd/gtfs