allow pip to be overriden in Makefile

Makefile

@@ -1,3 +1,5 @@
+PIP=pip
+
 clean:
 	rm -rf dist
 	rm -f cumulus-message-adapter.zip
@@ -6,7 +8,7 @@ dist:
 	mkdir -p dist
 
 requirements: dist requirements.txt
-	pip install -r requirements.txt --target ./dist/
+	$(PIP) install -r requirements.txt --target ./dist/
 	find ./dist -type d -name '__pycache__' | xargs rm -rf
 	find ./dist/botocore/data -mindepth 1 -maxdepth 1 -type d |\
 		egrep -v '(stepfunctions|s3)' |\