fix(client): print default string

When version is not specified in the arguments, you'd get a response:

    $ deis rollback
    Rolling back to vNone...

This makes it print a default string if no version is specified.

Author: Matthew Fisher

client/deis.py

@@ -1493,7 +1493,10 @@ def releases_rollback(self, args):
         else:
             body = {}
         url = "/api/apps/{app}/releases/rollback".format(**locals())
-        sys.stdout.write('Rolling back to v{version}... '.format(**locals()))
+        if version:
+            sys.stdout.write('Rolling back to v{version}... '.format(**locals()))
+        else:
+            sys.stdout.write('Rolling back one release... ')
         sys.stdout.flush()
         try:
             progress = TextProgress()