AE Datastore Geo: Add index for plain geo query.

The Travis build is failing without this.
https://travis-ci.org/GoogleCloudPlatform/java-docs-samples/builds/127329577

The docs at
https://cloud.google.com/appengine/docs/java/datastore/geosearch claim:

    Recall that "ordinary" (non-geospatial) queries require that an index
    exists that contains all the properties in the query and no others. This
    constraint is relaxed for geospatial queries.

But, this does not appear to be correct, at least not for the local
devserver.

appengine/datastore/geo/src/main/webapp/WEB-INF/datastore-indexes.xml

@@ -16,7 +16,10 @@
 <!-- The autoGenerate attribute can be true or false. We set it to false to -->
 <!-- allow verification that the index is correct in local tests.           -->
 <datastore-indexes autoGenerate="false">
-  <datastore-index kind="GasStation">
+  <datastore-index kind="GasStation" source="manual">
+    <property name="location" mode="geospatial" />
+  </datastore-index>
+  <datastore-index kind="GasStation" source="manual">
     <property name="brand" />
     <property name="location" mode="geospatial" />
   </datastore-index>