travis: Add some libsodium coverage

As far as I can tell we're not gating on this right now.  From
a quick glance, it looks like Debian stable has `libsodium-dev`
but only Ubuntu eoan does which we're not testing right now.

ci/travis-Dockerfile.in

@@ -2,7 +2,7 @@ FROM @ci_docker@
 ENV container docker
 
 ADD ci/travis-install.sh /travis-install.sh
-RUN ci_suite="@ci_suite@" ci_distro="@ci_distro@" ci_in_docker=yes /travis-install.sh
+RUN ci_suite="@ci_suite@" ci_distro="@ci_distro@" ci_pkgs="@ci_pkgs@" ci_in_docker=yes /travis-install.sh
 
 ADD . /home/user/ostree
 RUN chown -R user:user /home/user/ostree

ci/travis-install.sh

@@ -64,6 +64,7 @@ if [ -n "$ci_docker" ]; then
         -e "s/@ci_distro@/${ci_distro}/" \
         -e "s/@ci_docker@/${ci_docker}/" \
         -e "s/@ci_suite@/${ci_suite}/" \
+        -e "s/@ci_pkgs@/${ci_pkgs}/" \
         < ci/travis-Dockerfile.in > Dockerfile
     exec docker build -t ci-image .
 fi
@@ -111,6 +112,7 @@ case "$ci_distro" in
             procps \
             zlib1g-dev \
             python3-yaml \
+            ${ci_pkgs:-} \
             ${NULL}
 
         if [ "$ci_in_docker" = yes ]; then