use source_data from repmis

Hi 
Great work!

I created the `source_GitHubData` gist you originally used to download the data. I've since folded this functionality into a more general `source_data` function in the repmis package. This makes the code a lot simpler plus it is a stable CRAN based package.

These updates in the Demonstration.md file should work pretty well.
Merge pull request #4 from christophergandrud/patch-1

use source_data from repmis