Change from older :constraints to lambda syntax

Current syntax creates a deprecation warning when running tests. This smallest possible change fixes the issue by changing to the newer lambda syntax.

### Error message from test run

    DEPRECATION WARNING: The following options in your Doorkeeper::Application.has_many :authorized_tokens declaration are deprecated: :conditions. Please use a scope block instead. For exam ple, the following:

        has_many :spam_comments, conditions: { spam: true }, class_name: 'Comment'

    should be rewritten as the following:

        has_many :spam_comments, -> { where spam: true }, class_name: 'Comment'. (called from <top (required)> at /var/www/current/config/initializers/doorkeeper.rb:3)