Update version for 1.7.0-rc1 release

Signed-off-by: Anthony Liguori <aliguori@amazon.com>

VERSION

diff --git a/VERSION b/VERSION
index 86e63cc..56b4609 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-1.6.90
+1.6.91