Add support for 6.8

Author: Varun Rao Bhamidimarri

aws_emr_blog_v3/cloudformation/step2_ranger-rds-emr.template

@@ -234,6 +234,7 @@ Parameters:
       - emr-6.3.0
       - emr-6.4.0
       - emr-6.7.0
+      - emr-6.8.0
   S3Bucket:
     Description: S3Bucket for the code [Important!! for non US-East-1 regions the code needs to be in a regional bucket. Either pass the regional bucket name with the EMR bootstrap scripts and Lambda function code, or set 'CreateRegionalS3BucketAndCopyScripts' parameter to 'true']
     Type: String