Fix problem with helper_method for ruby less than 2.1

In ruby 2.1 `def` returns symbol, not nil. If I run my code with ruby 2.0 I got very strange error:
```
syntax error, unexpected *
def (*args, &blk)
      ^
(irb):1: syntax error, unexpected &
def (*args, &blk)
             ^
        from /usr/bin/irb:11:in `<main>'
```

My code is something like that:

```
helper_method def abc
	// CODE
end
```