Extract duty expression and description formatting

This change is for https://www.pivotaltracker.com/story/show/36718669 and partly covers https://www.pivotaltracker.com/story/show/58242830

Should be merged with pull request on backend app: https://github.com/alphagov/trade-tariff-backend/pull/122

Formatting was moved to backend app, so it can serve formatted data via API so that other clients wouldn't have to know TARIC rules to display the data.

Updated `therubyracer` to avoid compilation errors