Update version for v3.1.0-rc0 release

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
diff --git a/VERSION b/VERSION
index 929fd9f..ddd61cc 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-3.0.50
+3.0.90