travis: fix asan symbolizer path

Author: nunoplopes

contrib/ci/Dockerfiles/z3_base_ubuntu_18.04.Dockerfile

@@ -18,7 +18,7 @@ RUN apt-get update && \
         python-setuptools \
         sudo
 
-RUN curl -SL https://packages.microsoft.com/config/ubuntu/16.04/packages-microsoft-prod.deb --output packages-microsoft-prod.deb && \
+RUN curl -SL https://packages.microsoft.com/config/ubuntu/18.04/packages-microsoft-prod.deb --output packages-microsoft-prod.deb && \
     dpkg -i packages-microsoft-prod.deb && \
     apt-get update && \
     apt-get -y --no-install-recommends install dotnet-sdk-2.1
@@ -31,4 +31,4 @@ 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
+ENV ASAN_SYMBOLIZER_PATH=/usr/lib/llvm-7/bin/llvm-symbolizer