VOL-1460 Updated core to use voltha-protos instead of local protos
Moved protos to python directory in order to maintain functionaly of containers built there.
Added capability to do local builds of protos
Added instructions on running dep ensure for getting protos.
Updated github.com/golang/protobuf version to v1.3.1

Change-Id: Ia6ef55f07f0d5dcb5b750d7c37b21b71db85bfc4
diff --git a/Gopkg.toml b/Gopkg.toml
index 65255b0..796ff2a 100644
--- a/Gopkg.toml
+++ b/Gopkg.toml
@@ -61,7 +61,7 @@
 
 [[constraint]]
   name = "github.com/golang/protobuf"
-  version = "1.2.0"
+  version = "=1.3.1"
 
 [[constraint]]
   name = "github.com/google/uuid"
@@ -102,3 +102,7 @@
 [prune]
   go-tests = true
   unused-packages = true
+
+  [[prune.project]]
+    name = "github.com/opencord/voltha-protos"
+    unused-packages = false