[travis] hopefully fixed test build

locally tested with a xenial image, and this should work.

Unfortunately, cannot get the pygit2 to build against the provided
libgit2-dev, so using the packaged version instead.

.travis.yml

@@ -18,9 +18,10 @@ addons:
   apt:
     packages:
       libdbus-1-dev
+      libffi-dev
 install:
   - pip install -U coverage pytest pytest-mock freezegun
-  - pip install $(cat requirements/modules/*.txt | cut -d ' ' -f 1 | sort -u|grep -v dbus-python|grep -v libvirt-python)
+  - pip install $(cat requirements/modules/*.txt | cut -d ' ' -f 1 | sort -u|grep -v dbus-python|grep -v libvirt-python|grep -v Pillow|grep -v pygit2)
 script:
   - coverage run --source=. -m pytest tests -v
 after_script: