Move installing of deps to the docker container, instead of installing on the host machine on travis

Author: andrehjr

src/ci/docker/x86_64-gnu-llvm-6.0/Dockerfile

@@ -13,6 +13,8 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
   gdb \
   llvm-6.0-tools \
   libedit-dev \
+  libssl-dev \
+  pkg-config \
   zlib1g-dev \
   xz-utils