Update INSTALL.md with details of the AMI and install script

These instructions are heavily based on those written by me and
@dracos for FixMyStreet and MapIt.
Use a more recent rubygems from squeeze-backports

The version of rubygems in squeeze is old enough that it won't
work with our Gemfile / Gemfile.lock.  Fortunately, the one in
squeeze-backports seems to work OK.
Invoke commands as the unprivileged user in a login shell

On some setups, the unprivileged user's .bashrc file won't be
sourced unless you've started a login shell - we need that
to be sourced to add the gem's bin directory to the PATH,
or "bundle" won't be found, for example.
Reduce "backscatter" bounce risk by setting local_recipient_maps

Leaving the local_recipient_maps setting empty has a risk which is
described in the Postfix manual:

   "[...] That is, an empty value. With this setting, the Postfix SMTP
    server will not reject mail with "User unknown in local recipient
    table". Don't do this on systems that receive mail directly from
    the Internet. With today's worms and viruses, Postfix will become a
    backscatter source: it accepts mail for non-existent recipients and
    then tries to return that mail as "undeliverable" to the often
    forged sender address."

This commit changes the local_recipient_maps setting to only accept
(and potentially bounce) emails where the local part is known (one
that we've mentioned in general.yml) or to a Unix user that exists.

Fixes #1166
Rename /etc/postfix/regexp to /etc/postfix/transports

This seems slightly less confusing, since both this file
and /etc/postfix/recipients contain regexp patterns.  This
also switches to replacing any transport_maps file that was
in use and overwrites the existing file rather than modifying
it.
Add a Vagrantfile to use the install script in a Vagrant box

This should be used with:

  vagrant --no-color up

... which (after some time) will leave you with a running site at:

  http://alaveteli.10.10.10.30.xip.io

Many thanks to Simon Coffey (@urbanautomaton) and Chris Adams
(@mrchrisadams) who produced a working Vagrantfile with a
independent install script while our installation script work
was ongoing - their advice and that script were very helpful
for creating this Vagrantfile that uses our generic mySociety
installation script.
Set 'constraint_disabling: false' for tests in database.yml

Without this setting, the PostgreSQL user still has to be a superuser
for the tests to work without constraint disabling errors.