Switch Travis to Docker

As Travis only provides outdated packages (compilers in particular),
this changes the Travis configuration to use Docker images to build and
test the code. This also has the benefit that multiple distributions can
be tested and not just Ubuntu.

For the time being, Arch Linux and Ubuntu 18.04 are added as supported
platforms.

.ci/ci.sh

@@ -0,0 +1,7 @@
+#!/bin/bash
+
+git submodule update --init --recursive
+mkdir -p build/debug
+cd build/debug
+cmake ../.. -GNinja -DANTHEM_BUILD_TESTS=ON
+ninja anthem-app && ninja run-tests