VOL-1643 Update pyvoltha to use voltha-protos v3.0.0

It doesn't require any code changes other than updating the requirements file.

Change-Id: I87f3712a476dbda5798da1b124d521e4dc7b094f
diff --git a/VERSION b/VERSION
index 00355e2..bc4abe8 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-2.3.7
+2.3.8