Use 2.8.7 chart version of voltha-openolt-adapter for voltha-2.7 release

Change-Id: I6200a87c04556823b9eced88c82dfa9307d7dd76
diff --git a/releases/voltha-2.7 b/releases/voltha-2.7
index c5790d0..bbbd4a8 100644
--- a/releases/voltha-2.7
+++ b/releases/voltha-2.7
@@ -5,7 +5,7 @@
 
 export VOLTHA_CHART_VERSION=2.8.3
 export VOLTHA_BBSIM_CHART_VERSION=4.2.0
-export VOLTHA_ADAPTER_OPEN_OLT_CHART_VERSION=2.8.6
+export VOLTHA_ADAPTER_OPEN_OLT_CHART_VERSION=2.8.7
 export VOLTHA_ADAPTER_OPEN_ONU_CHART_VERSION=2.7.13 # needs update once released
 export BBSIM_SADIS_SERVER_CHART_VERSION=0.1.1
 export ONOS_CLASSIC_CHART_VERSION=0.1.13