Use Bundler instead of Jeweler for gem management

Bundler makes the process of building gems and bumping versions much simpler IMO. Instead of using a rake task to generate the gemspec file, the gemspec is edited directly. The version is always pulled from `version.rb`. Bumping a version is just a matter of editing that file. `rake -T` will show the tasks available for building the gem, but since we're not publishing the gem, we don't even need to do that.