Update switcher.json

Author: Conengmo

docs/_static/switcher.json

@@ -4,10 +4,14 @@
     "url": "https://python-visualization.github.io/folium/dev/"
   },
   {
-    "name": "latest (0.17.0)",
-    "version": "0.17.0",
+    "name": "latest (0.18.0)",
+    "version": "0.18.0",
     "url": "https://python-visualization.github.io/folium/latest/"
   },
+  {
+    "version": "0.17.0",
+    "url": "https://python-visualization.github.io/folium/v0.17.0/"
+  },
   {
     "version": "0.16.0",
     "url": "https://python-visualization.github.io/folium/v0.16.0/"