Pin Python package versions to the versions in the last successful build

The last successful build was
https://travis-ci.org/pi-hole/pi-hole/builds/500825408

When `testinfra` became `2.0.0`, it removed the deprecated features that
the tests currently use.

Signed-off-by: Mcat12 <newtoncat12@yahoo.com>

requirements.txt

@@ -1,6 +1,6 @@
-docker-compose
-pytest
-pytest-xdist
-pytest-cov
-testinfra
-tox
+docker-compose==1.23.2
+pytest==4.3.0
+pytest-xdist==1.26.1
+pytest-cov==2.6.1
+testinfra==1.19.0
+tox==3.7.0