bumping plek to 1.0.0

Also updating README to reflect the new breaking change where
govuk_setenv should be run prior to any testing. Tests that rely on
test.gov.uk have also been updated to dev.gov.uk.
bumping plek to 1.0.0

Also updating README to reflect the new breaking change where
govuk_setenv should be run prior to any testing. Tests that rely on
test.gov.uk have also been updated to dev.gov.uk.
swaping switch

Should not ideally be using  to change behaviour. Instead use  (if you have to)
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.