Add pinned versions of wheels and mkdocs to our dev requirements.

requirements.txt

@@ -16,3 +16,9 @@ django-filter>=0.5.4
 django-oauth-plus>=2.2.1
 oauth2>=1.5.211
 django-oauth2-provider>=0.2.4
+
+# wheel for PyPI installs
+wheel==0.24.0
+
+# MkDocs for documentation previews/deploys
+mkdocs==0.11.1