Take advantage of ruby 2.1's #def returns

From Ruby 2.1 onwards the def method returns a symbol created from the method's name, allowing us to avoid a bit of repetition when declaring private methods.

```ruby
## Ruby 2.0 < 
def foo; end
private :foo

## Ruby 2.1 > 
private def foo; end
```
This pull request shouldn't be merged until cuba stops supporting Ruby versions prior to 2.1, so I expect it to never be merged in practice. 

I was super bored though.