use requirements-dev

### What is the problem / feature ?

The readme didn't say to use `pip install -r requirements-dev.txt`

### How did it get fixed / implemented ?

Changed it to use `requirements-dev.txt`

### How can someone test / see it ?

Look at the readme

*Here is a cute animal picture for your troubles...*

![tumblr_m2nlww70jp1qe0inco1_500](https://cloud.githubusercontent.com/assets/824194/4553788/9b2f7aae-4ea0-11e4-8e66-288fd12c66fa.jpg)