Silence DEPRECATION WARNING due to eager loading

The .includes query method is meant for the eager loading of
associations. As of Rails 4, adding a string condition of the
included association results in a deprecation warning.

Add ActiveRecord::QueryMethods.references to "app/models/post.rb"
by passing ".references(:content)" which is a mandatory for
every post being submitted.
We want at least an editor being installed.

Add vim(1) to the basic development packages so we have
at least an editor installed on the virtual machine.

Sort the entries.
 Enable access to DNS from Vagrant guest machine.

 Add "--natdnshostresolver1" to Vagrantfile so that access
 to DNS from inside the vagrant guest is transparent.
 We don't want to set IP Addresses for "http_proxy" and the
 likes if needed.
Make guest machine reachable by Private Network IP.

This let's us access the Vagrant-machine using a static, known IP Address.
Adapt the reference in README.md accordingly.
Clarify online help and fix wording.
How to change the virtual host's IP Address.

Explain the parameter "config.vm.network" used to assign
the static IP Address to the virtual development host.