Updated README with installation instructions based on test-run 

@migurski said:

> I’ve re-written some of the installation instructions to better reflect existing resources we have for Ruby and PostgreSQL, and to highlight a few gotchas I’ve encountered along the way while testing installation on Ubuntu 14.04 Linux.

I incorporated @migurski's changes and updated the README to reflect more recent work related to making deploying easier (removed PostGIS requirement).

In the process of updating the documentation I noticed that we still had the shapefiles in the assets/ directory, and as those aren't needed I removed them.

This is a newer version of pull request #21, and this is ready to be merged, so I'll close that other one.