Update DEVELOPER-ADVANCED.md

To install the gem pg-0.15.1, you need to have libpq-dev installed. Otherwise bundle install will fail.
See http://stackoverflow.com/questions/16788062/error-running-gem-install-pg-v-0-15-1
Update DEVELOPER-ADVANCED.md

Merged installation instruction for libpq-dev into step 4.
Update DEVELOPER-ADVANCED.md

Fixed numbering of list items.
Merge pull request #2983 from fwoelm/patch-1

Update DEVELOPER-ADVANCED.md