travis: Switch back to autogenerated Docker images

Archlinux updated to PyQt 5.6 in [extra] now, so the autobuilt image
should work fine with Qt 5.6 now.

scripts/dev/ci/travis_run.sh

@@ -1,12 +1,7 @@
 #!/bin/bash
 
 if [[ $DOCKER ]]; then
-    # To build a fresh image:
-    # docker build -t img misc/docker/$DOCKER
-    # docker run --privileged -v $PWD:/outside img
-
-    docker run --privileged -v $PWD:/outside \
-        thecompiler/qutebrowser-manual:$DOCKER
+    docker run --privileged -v $PWD:/outside thecompiler/qutebrowser:$DOCKER
 else
     args=()
     [[ $TESTENV == docs ]] && args=('--no-authors')