Using latest pyvoltha in order to read kv-store-prefix from ENV

Change-Id: I8731750ebc03570b807fc49c46c6d64a30a2a748
diff --git a/python/requirements.txt b/python/requirements.txt
index 1bb982b..1050468 100644
--- a/python/requirements.txt
+++ b/python/requirements.txt
@@ -1,2 +1,2 @@
-voltha-protos==3.4.2
-pyvoltha==2.5.3
+voltha-protos==4.0.5
+pyvoltha==2.5.7