avocado.core.html: Add "update" hook

This hook is used by older pystache (pystache==0.4.1). We don't
use it so let's simply ignore it.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

avocado/core/html.py

@@ -56,6 +56,12 @@ class ReportModel(object):
         self.html_output = html_output
         self.html_output_dir = os.path.abspath(os.path.dirname(html_output))
 
+    def update(self, **kwargs):
+        """
+        Hook for updates not supported
+        """
+        pass
+
     def get(self, key, default):
         value = getattr(self, key, default)
         if callable(value):