Bump SparkR version string to 1.5.0

This patch is against master, but we need to apply it to 1.5 branch as well.

cc shivaram  and rxin

Author: Hossein <hossein@databricks.com>

Closes #8291 from falaki/SparkRVersion1.5.

R/pkg/DESCRIPTION

 Package: SparkR
 Type: Package
 Title: R frontend for Spark
-Version: 1.4.0
+Version: 1.5.0
 Date: 2013-09-09
 Author: The Apache Software Foundation
 Maintainer: Shivaram Venkataraman <shivaram@cs.berkeley.edu>