Moving VOLTHA-ONOS to depend on onos:2.2.6 and releasing 4.1.8

Change-Id: I3f0697eb9353d07eaded74f1f26334566296bdfa
diff --git a/VERSION b/VERSION
index 9edf2a4..a7c00da 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-4.1.7
+4.1.8