specify workflow build in requirements

requirements/base.txt

@@ -1,5 +1,5 @@
 Django>=3.0,<3.1
 django-dotenv==1.4.1
-wagtail>=2.8,<2.9
+http://matthew-tbx.s3.amazonaws.com/wagtail/wagtail-workflow20200224-2.8a0.tar.gz
 wagtailfontawesome>=1.1.3,<1.2
 Pillow>=6.2.0,<6.3