use clang 6

dockerfiles/xenial.docker

@@ -22,7 +22,7 @@ RUN apt-get update && \
     apt-get install -y libncurses5-dev && \
     apt-get install -y linux-headers-generic && \
     apt-get install -y graphviz && \
-    apt-get install -y clang && \
+    apt-get install -y clang-6.0 && \
     apt-get install -y libhdf5-dev && \
     apt-get install -y liblua5.2-dev && \
     apt-get install -y python-sphinx && \