Add missing dependency to Ubuntu image

For some reason, Bison is not implicitly installed along with the other
dependencies in the Ubuntu 18.04 image used for continuous integration.
This adds Bison explicitly.

.ci/Dockerfile-ubuntu-18.04

@@ -3,7 +3,7 @@ FROM ubuntu:18.04
 ARG toolchain
 
 RUN apt-get update
-RUN apt-get install -y cmake git ninja-build re2c
+RUN apt-get install -y bison cmake git ninja-build re2c
 RUN if [ "${toolchain}" = "gcc" ]; then apt-get install -y g++; fi
 RUN if [ "${toolchain}" = "clang" ]; then apt-get install -y clang; fi