[RELEASE] Released version 2.8-dev0

Released version 2.8-dev0 with the following main changes :
    - MINOR: version: mention that it's development again
diff --git a/VERSION b/VERSION
index 24ba9a3..3a18b82 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-2.7.0
+2.8-dev0