Update version for v2.7.0-rc1 release

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
diff --git a/VERSION b/VERSION
index 7d99c60..039b3b4 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-2.6.90
+2.6.91