ease off on momentjs-rails dependency

I'm currently working on a project using momentjs-rails v2.3.1 (using [this PR](https://github.com/derekprior/momentjs-rails/pull/17)), and the given dependency causes Bundler to bail out.

As far as I can tell, the changes in [Moment.js](https://github.com/moment/moment/#changelog) should not affect the DateRangePicker, but it's hard to be sure without tests (*hint hint* ;-))