Update clearance to 1.2.x

Even after removing the references to assign current_user we still get WARNINGS:
'Assigning the current_user this way has been deprecated. You should instead use the sign_in method.'

clearance 1.2.0 includes
https://github.com/thoughtbot/clearance/pull/376 which fixes the
`current_user=` on `sign_in_as`.
Merge pull request #683 from arthurnn/update_clearance

Update clearance to 1.2.x