Update description of scientific Python distributions

- The business model of Anaconda has changed slightly: now it's a free product with 'add-ons' that cost money.
- Enthought has largely replaced EPD with Canopy.
- I've moved Anaconda before Canopy: this is obviously subjective, but Anaconda includes a lot more for free (Canopy Express excludes many open source packages). Workshops like Software Carpentry now tend to point new users to Anaconda for installation.
Merge pull request #431 from takluyver/patch-1

Update description of scientific Python distributions