Fix Travis Tests

[grantal]

I made the tests pass on travis. Here is what I did:

• add django.contrib.sessions to to test/settings.py
• made test/urls.py import django_private_chat/urls.py without a namespace so that the reverse of dialog_detail in test_views.py would work
• made test_views.py inport reverse from django.core.urlresolvers rather than django.urls so that it will work in older versions of django
• made MessageMethodTest.test_str_method not enforce a specific format of string
• Stopped testing python2.7 since the websockets package does not have a python2.7 version
• Added python3.3 into tox.ini

[delneg]

Thanks!