Update version for v2.2.0-rc4 release

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

VERSION

diff --git a/VERSION b/VERSION
index 9be10bc..9b218e8 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-2.1.93
+2.1.94