Unbreak debian PHP packages install

library/php guys decided to break Debian PHP packages installation as per
docker-library/php#542.

This is a quick workaround to unblock the image build, but in the mid-run it
would be better to rethink the whole image build, using alpine, pecl installs,
eventually PHP7, etc.

Signed-off-by: pierrecdn <me@pierre-cheynier.net>

Author: pierrecdn

Dockerfile

@@ -1,12 +1,13 @@
 FROM php:5.6-apache
-MAINTAINER Pierre Cheynier <pierre.cheynier@sfr.com>
+MAINTAINER Pierre Cheynier <pierre.cheynier@gmail.com>
 
 ENV PHPIPAM_SOURCE https://github.com/phpipam/phpipam/archive/
 ENV PHPIPAM_VERSION 1.3.1
 ENV WEB_REPO /var/www/html
 
 # Install required deb packages
 RUN apt-get update && apt-get -y upgrade && \
+    rm /etc/apt/preferences.d/no-debian-php && \
     apt-get install -y php-pear php5-curl php5-mysql php5-json php5-gmp php5-mcrypt php5-ldap php5-gd php-net-socket libgmp-dev libmcrypt-dev libpng12-dev libfreetype6-dev libjpeg-dev libpng-dev libldap2-dev && \
     rm -rf /var/lib/apt/lists/*