diff --git a/.travis.yml b/.travis.yml
index 30b1164..943f5b5 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -2,8 +2,6 @@ language: python
 python:
   - "2.7"
 env:
-  global:
-    - PIP_DOWNLOAD_CACHE=$HOME/.pip_cache
   matrix:
     - TOX_ENV=coverage
     - TOX_ENV=flake8
@@ -15,7 +13,7 @@ env:
     - TOX_ENV=py34-django19
 cache:
   directories:
-    - $HOME/.pip-cache/
+    - $HOME/.cache/pip/
     - $HOME/build/nitmir/django-cas-server/.tox/
 install:
   - "travis_retry pip install setuptools --upgrade"
diff --git a/cas_server/tests/mixin.py b/cas_server/tests/mixin.py
index 246a1c6..ff83258 100644
--- a/cas_server/tests/mixin.py
+++ b/cas_server/tests/mixin.py
@@ -161,7 +161,8 @@ class XmlContent(object):
 
 class UserModels(object):
     """Mixin for test on CAS user models"""
-    def expire_user(self):
+    @staticmethod
+    def expire_user():
         """return an expired user"""
         client = get_auth_client()
 
@@ -172,7 +173,8 @@ class UserModels(object):
         ).update(date=new_date)
         return client
 
-    def get_user(self, client):
+    @staticmethod
+    def get_user(client):
         """return the user associated with an authenticated client"""
         return models.User.objects.get(
             username=settings.CAS_TEST_USER,
diff --git a/cas_server/tests/test_models.py b/cas_server/tests/test_models.py
index d2f999e..5b001a7 100644
--- a/cas_server/tests/test_models.py
+++ b/cas_server/tests/test_models.py
@@ -94,8 +94,8 @@ class TicketTestCase(TestCase, UserModels, BaseServicePattern):
         )
         models.ReplaceAttributName.objects.create(name="*", service_pattern=self.service_pattern)
 
+    @staticmethod
     def get_ticket(
-        self,
         user,
         ticket_class,
         service,
diff --git a/tox.ini b/tox.ini
index c2693d0..ad0fec7 100644
--- a/tox.ini
+++ b/tox.ini
@@ -15,11 +15,9 @@ exclude=migrations
 [base]
 deps =
     -r{toxinidir}/requirements-dev.txt
-passenv=PIP_DOWNLOAD_CACHE
 
 [testenv]
 commands=py.test {posargs:cas_server/tests/}
-passenv={[base]passenv}
 
 [testenv:py27-django17]
 basepython=python2.7
@@ -61,13 +59,10 @@ deps =
 basepython=python
 deps=flake8
 commands=flake8 {toxinidir}/cas_server
-passenv={[base]passenv}
 
 [testenv:coverage]
 basepython=python
-passenv=
-    CODACY_PROJECT_TOKEN
-    {[base]passenv}
+passenv=CODACY_PROJECT_TOKEN
 deps=
     -r{toxinidir}/requirements-dev.txt
     codacy-coverage