Pin python openolt protos to last non-TP version

Needed to work with a non-TP core.

Change-Id: I00952eafb1df4b95be7b0d7df626ca0402f24f06
diff --git a/python/requirements.txt b/python/requirements.txt
index 556e5e3..3bdf25f 100644
--- a/python/requirements.txt
+++ b/python/requirements.txt
@@ -59,5 +59,5 @@
 pexpect==4.6.0
 python-consul==0.6.2
 afkak==3.0.0.dev20181106
-voltha-protos>=0.1.4
+voltha-protos==0.1.4
 pyvoltha==0.2.2