[SPARK-10126] [PROJECT INFRA] Fix typo in release-build.sh which broke snapshot publishing for Scala 2.11

The current `release-build.sh` has a typo which breaks snapshot publication for Scala 2.11. We should change the Scala version to 2.11 and clean before building a 2.11 snapshot.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #8325 from JoshRosen/fix-2.11-snapshots.