Ruby :one:.:eight: compat
Choose Ruby 1.8 compatible gems until we drop 1.8 support
Bring Ruby 1.8 CI back
Do not include xpath as dependency

It will have bundler install capybara 0.3.9 and we can't run the specs with such an old version of capybara.
Use arel on GitHub so we can run the specs for the edge
Remove unneeded dependencies and fix sinatra specs for Ruby 1.8
Use a bit older database_cleaner to run the specs for mongoid 2.4