sinatra runner: pass the logger as a template local; remove from scope

This removes the `logger` method from the template scope and switches
to exposing it to the template as a local.  This is prep for moving
away from Sinatra for template rendering.  Not all template engines
will want to use a pre-defined scope so the scope is being deprecated.

@jcredding ready for review.
@jcredding this is no longer a nil/nil assertion b/c I update the fake sinatra call to actually add a logger to it's settings.