Switch to plain composer.

see https://drupal.org/node/2002114

we do this because we expect all the aegir packages to h ave the same
prefix, so that aptitude search aegir2 shows all packages

we conflict with the previous aegir-foo2 packages so we have an
upgrade path from previous 2.x releases, but the final 2.0 release
will drop those compatibility shims.

The problem with that approach is that it may break build
bots. Besides, if someone wants something else than postfix, they can
install it before Aegir is installed.

Issue #1800066 updated accordingly

This reverts commit cc6aeabc.

Conflicts:
	debian/control

apache2 remains the default

email sending should work regardless of the MTA used...

this is done partly in part of this complaint: #1800066

This reverts commit c3263284.

this will allows us to just do apt-get install aegir to upgrade all three packages

this also disables (chmod 0) the manual drush_make install so that
drush doesn't freak out on the duplicate code copy.

thanks to Jamie McClelland for spotting the missing unzip dependency