Update tox config file to accept Pipfile and to include Python 3.7

Author: JavierLuna

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py36
+envlist = py36,py37
 
 [testenv]
 commands =
@@ -16,10 +16,19 @@ whitelist_externals = /bin/bash
 
 [testenv:py36]
 basepython = python3.6
-deps = -rrequirements-tests.txt
-[testenv:py35]
-basepython = python3.5
-deps = -rrequirements-tests.txt
-[testenv:py27]
-basepython = python2.7
-deps = -rrequirements-tests.txt
+deps = pipenv
+commands=
+    pipenv install --dev --ignore-pipfile
+    python setup.py test
+setenv=
+    LC_ALL=en_GB.UTF-8
+    LANG=en_GB.UTF-8
+[testenv:py37]
+basepython = python3.7
+deps = pipenv
+commands=
+    pipenv install --dev --ignore-pipfile
+    python setup.py test
+setenv=
+    LC_ALL=en_GB.UTF-8
+    LANG=en_GB.UTF-8