use ruby 2.1.2 on travis

we'll be upgrading ruby to 2.1.2
upgrade to ruby 2.1.2

- upgrade `capybara` and `sass-rails`
Remove gemfury source from Gemfile as it's redundant
previous version of govuk_frontend_toolkit was yanked update to latest

Changes did not affect the trade-tariff-frontend
Update README
update capybara, poltergeist, phantomjs
Fix asset compilation issue in test by including the group

Also fixes rel external style after updates to govuk_frontend_toolkit
Use production slimmer asset host
Update gds-api-adapters
Fix colspan on no measures message
Cache bundled gems
clean up travis file
Merge pull request #140 from bitzesty/ruby-2.1.2

use ruby 2.1.2