Pin libjuju versions to 2.8.4

Change-Id: I10afc20b8e4b5099cbf3867b644b589702cabcf8
Signed-off-by: David Garcia <david.garcia@canonical.com>
diff --git a/docker/MON/Dockerfile b/docker/MON/Dockerfile
index a91b957..635577a 100644
--- a/docker/MON/Dockerfile
+++ b/docker/MON/Dockerfile
@@ -61,7 +61,7 @@
                           python3-n2vc${N2VC_VERSION}
 
 
-RUN python3 -m pip install -U "juju==2.8.2"
+RUN python3 -m pip install -U "juju==2.8.4"
 
 COPY scripts/ scripts/