Added ujson with fixed version.

nomad/normalizing/optimade.py

@@ -130,6 +130,4 @@ class OptimadeNormalizer(SystemBasedNormalizer):
             return True
 
         except Exception as e:
-            import traceback
-            traceback.print_exc()
             self.logger.warn('could not acquire optimade data', exc_info=e)

requirements.txt

@@ -11,6 +11,7 @@ Pint
 matid
 
 # infrastructue related
+ujson<2.0.0
 elasticsearch==6.4.0
 elasticsearch-dsl==6.4.0
 msgpack<0.6.0