Add clang to Travis configurations

This adds the clang compiler to the tested Travis configurations.

.ci/Dockerfile-arch-latest

@@ -1,6 +1,9 @@
 FROM archimg/base-devel:latest
 
+ARG toolchain
+
 RUN pacman -Sy
 RUN pacman -S --noconfirm boost cmake git ninja re2c
+RUN if [ "${toolchain}" = "clang" ]; then pacman -S --noconfirm clang; fi
 
 VOLUME /app