Upgrade to Wagtail 1.13

Specify with a range rather than a fixed version, so that we don't have to do this on every point release...

requirements/base.txt

@@ -3,6 +3,6 @@ django-dotenv==1.4.1
 # elasticsearch==2.x.x chosen for compatibility with t2.micro.elasticsearch and t2.small.elasticsearch
 # instance types on AWS (Elasticsearch 2.3). Adjust for your deployment as needed.
 elasticsearch==2.4.1
-wagtail==1.12.1
+wagtail>=1.13,<1.14
 wagtailfontawesome==1.0.6
 Pillow==4.0.0