[TravisCI] Fix getting proper stack traces for ASan/LSan. The

`llvm-symbolizer` tool needs to be installed and ASan/LSan needs
to be told where to find it.

contrib/ci/Dockerfiles/z3_base_ubuntu_16.04.Dockerfile

@@ -18,6 +18,7 @@ RUN apt-get update && \
         libgomp1 \
         libomp5 \
         libomp-dev \
+        llvm-3.9 \
         make \
         mono-devel \
         ninja-build \
@@ -35,4 +36,4 @@ RUN useradd -m user && \
     echo 'user  ALL=(root) NOPASSWD: ALL' >> /etc/sudoers
 USER user
 WORKDIR /home/user
-
+ENV ASAN_SYMBOLIZER_PATH=/usr/lib/llvm-3.9/bin/llvm-symbolizer