Manually select the last argument.

This implementation does not rely on Bash-specific functionality.
diff --git a/git-flow-release b/git-flow-release
index e24c55e..aea6e42 100644
--- a/git-flow-release
+++ b/git-flow-release
@@ -121,7 +121,7 @@
 	eval set -- "${FLAGS_ARGV}"
 
 	# read arguments into global variables
-	VERSION=${!#}
+	VERSION=$(last_arg "$@")
 	BRANCH=$PREFIX$VERSION
 }