gitlab-ci: Use LLVM 3.4 from Debian jessie for scons-llvm job

This gets us closer to the officially supported minimum version of LLVM,
which is 3.3.

Acked-by: Eric Engestrom <eric.engestrom@intel.com>

.gitlab-ci/debian-install.sh

@@ -19,9 +19,11 @@ add-apt-repository "deb https://apt.llvm.org/stretch/ llvm-toolchain-stretch-8 m
 
 sed -i -e 's/http:\/\/deb/https:\/\/deb/g' /etc/apt/sources.list
 echo 'deb https://deb.debian.org/debian stretch-backports main' >/etc/apt/sources.list.d/backports.list
+echo 'deb https://deb.debian.org/debian jessie main' >/etc/apt/sources.list.d/jessie.list
 
 apt-get update
 apt-get install -y -t stretch-backports \
+      llvm-3.4-dev \
       llvm-3.9-dev \
       libclang-3.9-dev \
       llvm-5.0-dev \