Some links used in Dockerfile.raspberrypi are not accessible anymore,
because they were pointing to old versions of some source packages and
new versions were released. This commit fixes this by changing the
links to point to the newest versions.
The RaspberryPi Dockerfile had some out-of-date dependency packages and
was downloading an outdated aria2 source from Sourceforge, which
resulted in a build for version 1.19.0. The new
fetch-latest-release-from-GitHub script I've cooked up should always
work as long as GitHub doesn't change their API and aria2's platform
agnostic releases follow the "{version number}.tar.bz2" naming format.
