Don't just take the last argument, take the first.

This patch was originally contributed as a workaround for the cases
where there were flags that took the first argument position.  This fix
was just plain wrong and this commit reverts it.
diff --git a/git-flow-release b/git-flow-release
index aea6e42..05815bc 100644
--- a/git-flow-release
+++ b/git-flow-release
@@ -121,7 +121,7 @@
 	eval set -- "${FLAGS_ARGV}"
 
 	# read arguments into global variables
-	VERSION=$(last_arg "$@")
+	VERSION=$1
 	BRANCH=$PREFIX$VERSION
 }