Always silence pull output with --parallel

This is how things were prior to the addition of the --quiet flag.
Making it not silent produces output that's weird and difficult to read.

Signed-off-by: Evan Shaw <[email protected]>

compose/project.py

@@ -496,7 +496,7 @@ class Project(object):
 
         if parallel_pull:
             def pull_service(service):
-                service.pull(ignore_pull_failures, silent=silent)
+                service.pull(ignore_pull_failures, True)
 
             _, errors = parallel.parallel_execute(
                 services,