Adds a missing dependency to Gitlab-CI

Author: StuntsPT

.gitlab-ci.yml

@@ -26,7 +26,7 @@ tests:
       - echo "deb http://cran.r-project.org/bin/linux/debian/ jessie-cran35/" >> /etc/apt/sources.list
       - apt-get update
       - apt-get install xz-utils
-      - apt-get install -y --force-yes r-base libcurl4-openssl-dev libssl-dev
+      - apt-get install -y --force-yes r-base libcurl4-openssl-dev libssl-dev libxml2 libxml2-dev
       - python -V               # Print out python version for debugging
       - pip install virtualenv
       - virtualenv venv