slides: remove significant trailing whitespace in code blocks

The trailing newlines cause each of the code blocks to have an empty line at the bottom. This breaks the illusion that the code is on a white surface which sits in front of the background and casts a shadow on it:

![glitch](https://cloud.githubusercontent.com/assets/210406/4429979/214a2686-4611-11e4-9d3f-526e01b152ea.png)