Database migrations

@maxfliri
Add index on collect_at for artefacts

@maxfliri
Migrations support moved to recorder gem

@maxfliri
Run migrations in jenkins.sh

@gtrogers
@maxfliri
Merge pull request #2 from alphagov/migration

Migrations + Index on artefacts.collected_at
Force migrations on test db in jenkins.sh

@maxfliri
@gtrogers