bumping plek to 1.0.0

And updating any tests to use dev.gov.uk instead of test.gov.uk
bumping slimmer

This is to bring in the change from Plek.current.find('assets') to Plek.current.find('static') such that slimmer becomes compatible with plek 1.0.0 (as a transient dependency) as well as old plek.
Bump plek to 1.1.0

prevents tests compaining about GOVUK_APP_DOMAIN
Merge remote branch 'origin/bump-plek'