Update pytest-xdist to 1.16.0 (#105)

* Update pytest-xdist from 1.15.0 to 1.16.0

* Update pytest-xdist from 1.15.0 to 1.16.0

requirements/py27-tests.txt

@@ -3,4 +3,4 @@ mozlog==3.4
 pytest==3.0.7
 pytest-metadata==1.3.0
 pytest-selenium==1.9.1
-pytest-xdist==1.15.0
+pytest-xdist==1.16.0

requirements/py36-tests.txt

@@ -2,4 +2,4 @@ mock==2.0.0
 pytest==3.0.7
 pytest-metadata==1.3.0
 pytest-selenium==1.9.1
-pytest-xdist==1.15.0
+pytest-xdist==1.16.0