removed tox from setup

Makefile

@@ -43,7 +43,6 @@ clean-pyc: ## remove Python file artifacts
 	find . -name '__pycache__' -exec rm -fr {} +
 
 clean-test: ## remove test and coverage artifacts
-	rm -fr .tox/
 	rm -f .coverage
 	rm -fr htmlcov/
 
@@ -52,10 +51,6 @@ lint: ## check style with flake8
 
 test: ## run tests quickly with the default Python
 	py.test
-	
-
-test-all: ## run tests on every Python version with tox
-	tox
 
 coverage: ## check code coverage quickly with the default Python
 	coverage run --source ordr2 -m pytest

requirements_dev.txt

@@ -1,11 +1,9 @@
-pip==8.1.2
-bumpversion==0.5.3
-wheel==0.29.0
-watchdog==0.8.3
-flake8==2.6.0
-tox==2.3.1
-coverage==4.1
-Sphinx==1.4.8
-
-pytest==2.9.2
-pytest-runner==2.11.1
+pip>=8.1.2
+bumpversion>=0.5.3
+wheel>=0.29.0
+watchdog>=0.8.3
+flake8>=2.6.0
+coverage>=4.1
+Sphinx>=1.4.8
+pytest>=2.9.2
+pytest-runner>=2.11.1