Bumping minor version after voltha-2.7 release (2.7.0 has been tagged on the voltha-2.7 branch)

Change-Id: I3d3d58acafa777a908cc3dbf39c4d62261e4d2e3
diff --git a/VERSION b/VERSION
index 6886b8a..9781df7 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-2.7.0-dev
+2.8.0-dev