Use latest version of pip when running tox

Author: madhuBayy

requirements/requirements-test.txt renamed to requirements-test.txt

@@ -5,9 +5,11 @@ envlist = py36, py37, py37-flake8
 max-line-length = 99
 
 [testenv]
-deps = -rrequirements/requirements-test.txt
+deps = -rrequirements-test.txt
+       pip >= 18.0.1
 commands = pytest
 
 [testenv:py37-flake8]
 deps = flake8
+       pip >= 18.0.1
 commands = flake8 aubreylib tests setup.py