Update to DateRangePicker v1.3.12

The lack of a dependency version for the `momentjs-rails` gem in the README is causing issues. MomentJS is now on 2.8.1 and has breaking changes with the current, outdated version of the DateRangePicker. This update to the latest version of DateRangePicker overcomes those issues, restoring functionality without forcing users to install an older version of the `momentjs-rails` gem.

I've also fixed a small bug with the example code in the README.