Moving version to 2.8.0-dev (2.7.0 has been released on the voltha-2.7 branch)

Change-Id: I603494eef1c5fc3e06fd2a6708638e4871b04ba8
diff --git a/VERSION b/VERSION
index 6886b8a..9781df7 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-2.7.0-dev
+2.8.0-dev