Override ruby-end-insert-newline to t if RET is pressed

At the moment the combination of `ruby-end-expand-on-ret t` and `ruby-end-insert-newline nil` leads to uncomfortable situations when after expansion, cursor is just before the automatically inserted `end`. There's no quick way to go on to writing the class (or method) body, you have to do something like `C-j`, (EDIT:) `C-p`, `TAB`.

This commit fixes that.

Maybe we should update the `ruby-end-insert-newline` docstring, but I'm not sure how. Technically, that newline is not "additional" in this case.