Use venv & run release-test [ci skip]

- Call python with relative path (bin/python) to use venv python.
- Possibly positioning `install-venv` to be the default target.
- No need to factor out venv target.

Makefile

@@ -55,7 +55,10 @@ install:
 install-req:
 	pip install -r requirements.txt
 
-install-venv: venv install-req
+install-venv: 
+	virtualenv .
+	bin/pip install -r requirements.txt
+	$(MAKE) release-test
 
 release-test:
 	$(MAKE) install-req
@@ -85,8 +88,5 @@ upload-test:
 upload:
 	python setup.py sdist --format=gztar,zip upload
 
-venv:
-	virtualenv .
-
 readme:
 	viewdoc