Enforce available locales

Do not disable config.i18n.enforce_available_locales
Add delegate for screenshot URL to Webpage
Call Rails.application instead
Use hybrid cookies serializer
Precompile Teaspoon assets
Upgrade to RSpec 3.0

- Tag spec example groups with 'type' metadata
- Use vcr_opts hash to store 'vcr' metadata for spec example groups
- Change example group describe to RSpec.describe
- Change should to is_expected.to
- Change be_true to be true and be_false to be false
- Replace stub() with allow().to receive
- Explicitly define city and country methods
- Upgrade rspec-rails to 3.0
- Separate Rails-specific logic into rails_helper