SPARK-4910 [CORE] build failed (use of FileStatus.isFile in Hadoop 1.x)

Fix small Hadoop 1 compile error from SPARK-2261. In Hadoop 1.x, all we have is FileStatus.isDir, so these "is file" assertions are changed to "is not a dir". This is how similar checks are done so far in the code base.

Author: Sean Owen <sowen@cloudera.com>

Closes #3754 from srowen/SPARK-4910 and squashes the following commits:

52c5e4e [Sean Owen] Fix small Hadoop 1 compile error from SPARK-2261