Dockerfile: fix

Author: chathhorn

Dockerfile

@@ -7,6 +7,9 @@ FROM runtimeverificationinc/kframework:ubuntu-bionic
 RUN   apt-get update -q     \
   &&  apt-get install --yes \
         libstdc++6          \
+        llvm-6.0            \
+        clang++-6.0         \
+        clang-6.0           \
         clang-8             \
         libclang-8-dev      \
         llvm-8-tools        \