ci: Use gdc instead of gdc-6

gdc-6 is currently not in testing and gdc will pull in the default version
(currently gdc-8) which should work.

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>

ci/Dockerfile

@@ -34,7 +34,7 @@ RUN apt-get -y install ca-certificates && \
       g++-7 \
       gcc-7 \
       gdb \
-      gdc-6 \
+      gdc \
       git \
       golang-go \
       kbd \