[DO NOT MERGE] 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.
The bin `bundle` doesn't, but `bundler/setup` will - but I don't believe that effects this decision.

The bin `bower` will be run in development and CI, so we should be OK to move to the development group.