Move version to a separate file for Heroku

Heroku will not allow me to bundle install venice without it screaming: 

> Does it try to require a relative path? That's been removed in Ruby 1.9.

This is because the gemspec is requiring the whole gem just to get the version, which is requiring all sorts of other gems.  I've just moved the version to a different, separate file, and Heroku loves it again.

By the way, thanks a million for doing the recurring subscription work, it's awesome!