Hopefully temporary version cap for deform

getting the following error w/ v2.0.15:

```
  File "/srv/envs/XXX/lib/python3.7/site-packages/deform/field.py", line 452, in get_widget_requirements
    requirements = [req for req in self.widget.requirements] + [
TypeError: 'NoneType' object is not iterable
```

setup.py

@@ -76,9 +76,11 @@ requires = [
     # TODO: remove version cap once we can drop support for python 2.x
     'cornice<5.0',                      # 3.4.2                 4.0.1
 
+    # TODO: remove once their bug is fixed?  idk what this is about yet...
+    'deform<2.0.15',                    # 2.0.14
+
     'colander',                         # 1.7.0
     'ColanderAlchemy',                  # 0.3.3
-    'deform',                           # 2.0.4
     'humanize',                         # 0.5.1
     'Mako',                             # 0.6.2
     'openpyxl',                         # 2.4.7