SPARK-1518: FileLogger: Fix compile against Hadoop trunk

In Hadoop trunk (currently Hadoop 3.0.0), the deprecated
FSDataOutputStream#sync() method has been removed.  Instead, we should
call FSDataOutputStream#hflush, which does the same thing as the
deprecated method used to do.

Author: Colin McCabe <cmccabe@cloudera.com>

Closes #898 from cmccabe/SPARK-1518 and squashes the following commits:

752b9d7 [Colin McCabe] FileLogger: Fix compile against Hadoop trunk
(cherry picked from commit 1765c8d0ddf6bb5bc3c21f994456eba04c581de4)

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