Set -u after setting default for DEPLOY_TO

Currently DEPLOY_TO is accidentally required, not optional
Make DEPLOY_TO required

Now that old preview is dead, the DEPLOY_TO env var must be set in all
our environments, so a fallback should not be necessary. This makes
DEPLOY_TO required and bails if it is not set.
Merge pull request #94 from alphagov/fix-set-u

Set -u after setting default for DEPLOY_TO