SPARK-1846 Ignore logs directory in RAT checks

https://issues.apache.org/jira/browse/SPARK-1846

Author: Andrew Ash <andrew@andrewash.com>

Closes #793 from ash211/SPARK-1846 and squashes the following commits:

3f50db5 [Andrew Ash] SPARK-1846 Ignore logs directory in RAT checks
(cherry picked from commit 3abe2b734a5578966f671c34f1de34b4446b90f1)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>