[fix] Explicitely install postfix
[fix] Explicitely install postfix

On systems where exim4 is installed, the installation of yunohost package fails with the following error message :

Reading package lists...
Building dependency tree...
Reading state information...
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 yunohost : Depends: postfix-ldap but it is not going to be installed
            Depends: postfix-policyd-spf-perl but it is not going to be installed
            Depends: postfix-pcre but it is not going to be installed
            Recommends: ntp but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

Explicitely requesting an installation of postfix fixes the issue and triggers the removal of exim4.
YunoHost/install_script,bleuchtang/install_script