fix(ui-grid): fix isRowSelectable typing (#9594)

* per [the ui-grid
  documentation](http://ui-grid.info/docs/#/tutorial/210_selection),
  `isRowSelectable` is a function that returns a boolean, rather than a straight boolean property.

ui-grid/ui-grid-tests.ts

@@ -111,7 +111,7 @@ gridApi.core.clearAllFilters(true);
 gridApi.core.addToGridMenu(gridInstance, [menuItem]);
 gridApi.core.getVisibleRows(gridInstance);
 gridApi.core.handleWindowResize();
-gridApi.core.queueGridRefresh()
+gridApi.core.queueGridRefresh();
 gridApi.core.queueRefresh();
 gridApi.core.registerColumnsProcessor(colProcessor, 100);
 
@@ -122,7 +122,9 @@ var gridOptions: uiGrid.IGridOptionsOf<IMyEntity> = {
             console.log(row.entity.name);
         })
     }
-}
+};
+gridOptions.isRowSelectable = () => true;
+
 interface IAnotherEntity {
     anObject: string
 }

ui-grid/ui-grid.d.ts

@@ -2825,7 +2825,7 @@ declare namespace uiGrid {
              * Makes it possible to specify a method that evaluates for each row and sets its "enableSelection"
              * property.
              */
-            isRowSelectable?: boolean;
+            isRowSelectable?: (row: IGridRow) => boolean;
             /**
              * Enable multiple row selection only when using the ctrlKey or shiftKey. Requires multiSelect to be true.
              * Defaults to false