Mount PensionsCalculator (behind a feature flag)

* introduces bowndler for bower/bundler integration
* references pensions_calculator branch on github until pensions_calculator released gem is ready to be mounted

This PR should only be merged once the pensions_calculator is hooked in from a built gem.
So an offline conversation between @jongilbraith and I lead us to https://github.com/rails/rails/blob/master/actionview/lib/action_view/helpers/rendering_helper.rb#L30

If you pass a block to "render" in a rails view, than the layout is *always* rendered as a partial, which means the lookup rules with underscores and partial names apply.

Sad face.