Temporarily replaced Travis tests with python3.5 and 3.7 by python3.6
Temporarily replaced Travis tests with python3.5 and 3.7 by python3.6

hynekcer/django-salesforce,hynekcer/django-salesforce,django-salesforce/django-salesforce,django-salesforce/django-salesforce,hynekcer/django-salesforce,django-salesforce/django-salesforce