Version bump to Wagtail 2.6

requirements/base.txt

@@ -1,5 +1,5 @@
 Django>=2.1,<2.2
 django-dotenv==1.4.1
-wagtail>=2.5,<2.6
+wagtail>=2.6,<2.7
 wagtailfontawesome>=1.1.3,<1.2
 Pillow==4.0.0