Add requirement for coverage module

Makefile

@@ -31,8 +31,5 @@ setup: install migrate
 setup_ci:
 	pip install -U -r requirements/build.txt
 
-develop:
-	pip install -U -r requirements/dev.txt
-
 superuser:
 	python InvenTree/manage.py createsuperuser

requirements/build.txt

@@ -1,2 +1,3 @@
 -r base.txt
 flake8==3.3.0
+coverage>=4.5.3