Check that all que migrations have been applied at startup

[jpaulgs]

We maintain a CURRENT_VERSION of the que schema in lib/que/migrations.rb but we do not appear to enforce that all migrations have been applied on startup.

Let's fail loudly if migrations have not been applied.

[jpaulgs]

We could add an option / environment variable to allow this check to be bypassed, if for some reason a user of que required that...