Build against ubuntu xenial (#123)

Fixes #98

Dockerfile

@@ -1,6 +1,8 @@
 # Use a relatively old/stable distro here to maximize the supported platforms
 # and avoid depending on more recent version of, say, libc.
-FROM debian:stretch
+# Here we choose Xenial 16.04 which mean we also support Debian from stretch
+# (releases 2017) onwards.
+FROM ubuntu:xenial
 
 RUN apt-get update \
  && apt-get install -y --no-install-recommends \