Fix Debian 7 Travis tests

Debian wheezy image has been moved to debian/eol (https://hub.docker.com/r/debian/eol)
Update .travis/debian7/Dockerfile to use this image.

Author: bourtemb

.travis/debian7/Dockerfile

@@ -1,15 +1,11 @@
-FROM debian:wheezy-backports
+FROM debian/eol:wheezy
 
 ARG APP_UID=2000
 
 ARG APP_GID=2000
 
 MAINTAINER TANGO Controls team <tango@esrf.fr>
 
-RUN echo "deb http://archive.debian.org/debian wheezy main contrib" > /etc/apt/sources.list
-
-RUN echo "deb http://security.debian.org/debian-security wheezy/updates main" > /etc/apt/sources.list.d/security.list
-
 RUN echo 'deb http://archive.debian.org/debian wheezy-backports main contrib' > /etc/apt/sources.list.d/backports.list
 
 RUN echo "Acquire::Check-Valid-Until false;" > /etc/apt/apt.conf