Fix requirements

>= rather than ==

requirements/base.txt

@@ -1,9 +1,9 @@
-Django==1.11
-pillow==3.1.2
-djangorestframework==3.6.2
-django_filter==1.0.2
-django-simple-history==1.8.2
-coreapi==2.3.0
-pygments==2.2.0
-django-crispy-forms==1.7.2
-django-import-export==1.0.0
+Django>=1.11
+pillow>=5.0.0
+djangorestframework>=3.6.2
+django_filter>=1.0.2
+django-simple-history>=1.8.2
+coreapi>=2.3.0
+pygments>=2.2.0
+django-crispy-forms>=1.7.2
+django-import-export>=1.0.0