Updated README with installation instructions based on test-run

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 am unable to proceed past the final `bundle` step:

```
$ bundle exec rake council_districts:load

  CouncilDistrict Load (1.2ms)  SELECT "council_districts".* FROM "council_districts"
File contains 6 records
rake aborted!
RGeo::Error::RGeoError: GEOS is not available, but is required for correct interpretation of polygons in shapefiles.
/home/migurski/.rvm/gems/ruby-2.1.1/gems/rgeo-shapefile-0.2.3/lib/rgeo/shapefile/reader.rb:623:in `_read_polygon'
/home/migurski/.rvm/gems/ruby-2.1.1/gems/rgeo-shapefile-0.2.3/lib/rgeo/shapefile/reader.rb:453:in `_read_next_record'
/home/migurski/.rvm/gems/ruby-2.1.1/gems/rgeo-shapefile-0.2.3/lib/rgeo/shapefile/reader.rb:406:in `each'
/home/migurski/MuniciPal/lib/tasks/council_districts.rake:13:in `block (3 levels) in <top (required)>'
/home/migurski/.rvm/gems/ruby-2.1.1/gems/rgeo-shapefile-0.2.3/lib/rgeo/shapefile/reader.rb:207:in `open'
/home/migurski/MuniciPal/lib/tasks/council_districts.rake:11:in `block (2 levels) in <top (required)>'
/home/migurski/.rvm/gems/ruby-2.1.1/bin/ruby_executable_hooks:15:in `eval'
/home/migurski/.rvm/gems/ruby-2.1.1/bin/ruby_executable_hooks:15:in `<main>'
Tasks: TOP => council_districts:load
(See full trace by running task with --trace)
```