Jenkins: Update Python UI-test dependencies (#2763)

tests/jenkins/requirements.txt

@@ -1,7 +1,7 @@
 # pyup: ignore file
 mozlog==3.5
-PyPOM==1.1.1
-pytest==3.1.2
+PyPOM==1.2.0
+pytest==3.2.2
 pytest-metadata==1.5.0
-pytest-selenium==1.11.0
-pytest-xdist==1.18.0
+pytest-selenium==1.11.1
+pytest-xdist==1.20.0