Pin the version of libvirt-python to < 4.1.0

Version 4.1.0 is incompatible with older libvirt versions, so install a
version older than that.

.travis.yml

@@ -12,11 +12,12 @@ install:
     - sudo apt-get --force-yes install g++-4.8
     - sudo apt-get install rpm linux-headers-$(uname -r)
     - git clone https://github.com/draios/sysdig.git ../sysdig
-    - sudo apt-get install -y python-pip libvirt-dev libvirt-python jq dkms
+    - sudo apt-get install -y python-pip libvirt-dev jq dkms
     - cd ..
     - curl -Lo avocado-36.0-tar.gz https://github.com/avocado-framework/avocado/archive/36.0lts.tar.gz
     - tar -zxvf avocado-36.0-tar.gz
     - cd avocado-36.0lts
+    - sed -e 's/libvirt-python>=1.2.9/libvirt-python>=1.2.9,<4.1.0/' < requirements.txt  > /tmp/requirements.txt && mv /tmp/requirements.txt ./requirements.txt
     - sudo -H pip install -r requirements.txt
     - sudo python setup.py install
     - cd ../falco