Add migrations as require-dev
Better use require here.
Merge pull request #192 from cakephp/master-migrations

Add migrations as "require" composer dependency
Catch missing bake and debug kit plugins

Otherwise the app won't actually work in production, without either the
development-plugins being added or user modifying their bootstrap files.
(users _should_ modify their bootstrap files - but it's likely a
significant number won't and be confused by the missing plugin errors if
they remove the development-only plugins).
Change to debug-mode swith for DebugKit
Remove unused exception
Move debug kit from require to require-dev