Update pbr from 3.0.1 to 3.1.1 (#24)

Author: pyup-bot

{{cookiecutter.project_slug}}/requirements.txt

@@ -16,7 +16,7 @@ MarkupSafe==1.0
 mock==2.0.0
 parse==1.8.2
 parse-type==0.3.4
-pbr==3.0.1
+pbr==3.1.1
 pluggy==0.4.0
 py==1.4.34
 pycodestyle==2.3.1