Cleanup gemspec warnings & add missing 3.4.1 version

* Update the gemspec version to 3.4.1 (which is the version listed on Rubygems right now)
* Remove gem dependencies that are required by activerecord/activesupport anyway (cleans up `gem build` warnings)
* Lockdown most of the dependencies from major-version upgrades (safer, and cleans up `gem build` warnings)

(Rails 4.1 was released this week too, so it was upgraded in `Gemfile.lock`, which we might want to ignore eventually anyway)