Update pytest-xdist from 1.17.1 to 1.18.0 (#95)

Author: pyup.io bot

tests/requirements.txt

@@ -12,7 +12,7 @@ pytest-mock==1.6.0
 pytest-sugar==0.8.0
 pytest-timeout==1.2.0
 pytest-toolbox==0.2
-pytest-xdist==1.17.1
+pytest-xdist==1.18.0
 Sphinx==1.6.2
 
 # required to run "start" apps