fix(doc): Update README.md

Now spark 2.0 branch is merged to master. Update README to reflect this in version matrix table.

Author: noorul

README.md

@@ -114,7 +114,7 @@ Spark Job Server is now included in Datastax Enterprise 4.8!
 | 0.6.1       | 1.5.2         |
 | 0.6.2       | 1.6.1         |
 | 0.7.0       | 1.6.2         |
-| spark-2.0-preview | 2.1.0    |
+| 0.8.0-SNAPSHOT | 2.1.0    |
 
 For release notes, look in the `notes/` directory.