Update pip from 18.0 to 18.1

pyup-bot · pyup-bot · commit 440a375184fa · 2018-10-05T22:03:29.000+08:00
diff --git a/requirements_dev.txt b/requirements_dev.txt
@@ -1,4 +1,4 @@
-pip==18.0
+pip==18.1
 -e .
 
 bumpversion==0.5.3
