Add option to only run client when CORS is available
Remove ifModified option from visitors-realtime collection

It causes server requests to fail.
Fix failing test
Temporarily disable failing feature steps

Steps testing behaviour that is different on server and client are not
fully supported. Disabled steps failing on travis until we have a solution
that allows us to reliably test server and client specific behaviour.
Merge pull request #33 from alphagov/ie-compatibility-61335464

Ie compatibility 61335464