Fix syntax to be compatible with Ruby 1.8

  $ git ls-files *.rb* | xargs -n1 ruby -c
  Syntax OK
  Syntax OK
  lib/hiera/backend/aws/rds.rb:37: syntax error, unexpected '.', expecting
  kEND
	      .select { |i| i[:db_instance_status] == "available" }
	       ^
  Syntax OK
  Syntax OK
  Syntax OK
  Syntax OK
  Syntax OK
  Syntax OK
  Syntax OK

Thanks to @zined.