RDS engine version ignored

[johnwards]

When I setup my RDS instance as below I end up with an instance running MySQL 5.6.x rather than the 5.5.27 I requested. I checked and I can manually create an instance using this MySQL version fine.

resource "aws_db_instance" "default" {
    identifier = "mydb-terraform-rds"
    allocated_storage = 20
    engine = "mysql"
    engine_version = "5.5.27"
    instance_class = "db.m1.small"
    name = "dbname"
    username = "dbuser"
    password = "apassword"
    security_group_names = ["${aws_db_security_group.default.name}"]
    multi_az = true
    backup_retention_period = 7
    backup_window = "03:30-04:30"
    skip_final_snapshot = true
}