Change make venv to use standard python binary (fixes Travis-CI not using correct Python version in each test environment)

Install appropriate version of virtualenv for python, fixes Python 3.6 build because Travis-CI comes with virtualenv 12.0.1 by default

Author: ilanbiala

.travis.yml

@@ -7,7 +7,8 @@ python:
   - "3.5"
   - "3.6"
 install:
+  - pip install virtualenv --upgrade
   - make install
   - make test-install
-script: 
+script:
   - make test

Makefile

@@ -1,7 +1,7 @@
 .PHONY: clean install analysis test test-install develop docs docs-install
 
 venv:
-	virtualenv venv
+	virtualenv --python=python venv
 
 install: venv
 	. venv/bin/activate; pip install .