Don't propagate log messages from our custom logger to the root logger.
Don't propagate log messages from our custom logger to the root logger.

The propagation was causing all log messages to be printed twice.  I
don't know why, but I tracked it down to build.kake.server_client
calling logging.info() which calls basicConfig() which apparently adds
a stream to the default logger.  And it seems that the way propagation
works by default, the default logger gets the default-log-level of our
logger, instead of INFO which it was initialized to.
(server_client.py is being called because one of our linters imports
the whole world.)

Anyway, we can resolve all this by turning off propagation.

Auditors: benkraft, jacqueline

Test plan:
I ran
   ka-lint appengine/stats.py
with a few lint errors.  The output is now:
```
appengine/stats.py:11:1: F401 'gc' imported but unused
appengine/stats.py:22:1: F401 'google.appengine.api.quota' imported but unused
appengine/stats.py:23:1: F401 'google.appengine.api.runtime' imported but unused
appengine/stats.py:28:1: F401 'shared.shared_globals' imported but unused
appengine/stats.py:32:1: F401 'appengine.modules_util' imported but unused
appengine/stats.py:35:1: F401 'lib.ka_globals' imported but unused
```
instead of things like:
```
DEBUG:khan-linter:appengine/stats.py:11:1: F401 'gc' imported but unused
appengine/stats.py:11:1: F401 'gc' imported but unused
DEBUG:khan-linter:appengine/stats.py:22:1: F401 'google.appengine.api.quota' imported but unused
appengine/stats.py:22:1: F401 'google.appengine.api.quota' imported but unused
...
```

Khan/khan-linter,Khan/khan-linter,Khan/khan-linter,Khan/khan-linter