docker: Install Python 3.4

docker: Install Python 3.4

Currently, we tox test against Python 3.4. Python 3.4 is included with
Ubuntu 14.04, which is supported for several years to come, so we want
to keep supporting it. However, Python 3.4 isn't included with Ubuntu
16.04, which is what the Docker image is based on.

We could downgrade the container to 14.04, but then we'd struggle to
get Python 3.5 into the container. Python 3.5 is the most recent in the
Python 3 series, so we should also be supporting it.

Add the apt sources for Trusty and pull in Python 3.4 from there.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Stephen Finucane <stephenfinucane@hotmail.com>