Another missing package

Signed-off-by: Michal Čihař <michal@cihar.com>

.scrutinizer.yml

@@ -16,6 +16,6 @@ checks:
         code_rating: true
         duplicate_code: true
 before_commands:
-    - sudo apt-get install git libffi-dev libicu-dev libxslt1-dev
+    - sudo apt-get install git libffi-dev libicu-dev libxslt1-dev libxml2-dev
     - pip install -r requirements.txt
     - pip install -r requirements-optional.txt