Upgrade to Devise 2.2.3 for security fix

devise_invitable 1.1.0 was incompatible, and upgrading appeared to fix it.
Disable test that depends on old Devise behaviour

We will revisit after we've released the security upgrade and see if we want to
reinstate the behaviour.
Remove commented line
Merge pull request #53 from alphagov/upgrade_devise_to_2.2.3

Upgrade devise to 2.2.3