Improve Development Environment --> CocoaPods section with Ruby bundler information

[tylermilner]

We should add information about using Ruby bundler to manage the version of CocoaPods used for a project. The information about setting up your Ruby environment properly using rbenv is still relevant, but it would technically mean that you don't need to globally gem install cocoapods in order to build projects (you would instead do bundle install and bundle exec pod install for each project directory you encounter).