Fix ./pants server output

- Fix floating point millis in run name, which was breaking the `./pants server` output

good:
  pants_run_2015_04_11_15_55_57_967
bad:
  pants_run_2015_04_11_15_57_16_270.554931641

Testing Done:
verified that the server reports logs

Reviewed at https://rbcommons.com/s/twitter/r/2067/

src/python/pants/goal/run_tracker.py

@@ -80,7 +80,7 @@ def __init__(self,
     cmd_line = ' '.join(['./pants'] + sys.argv[1:])
 
     # run_id is safe for use in paths.
-    millis = (self.run_timestamp * 1000) % 1000
+    millis = int((self.run_timestamp * 1000) % 1000)
     run_id = 'pants_run_{}_{}'.format(
                time.strftime('%Y_%m_%d_%H_%M_%S', time.localtime(self.run_timestamp)), millis)