use versioned erlang binary

Index: vendor/relx/priv/templates/bin
--- vendor/relx/priv/templates/bin.orig
+++ vendor/relx/priv/templates/bin
@@ -26,7 +26,7 @@ find_erts_dir() {
     if [ -d "$__erts_dir" ]; then
         ERTS_DIR="$__erts_dir";
     else
-        __erl="$(command -v erl)"
+        __erl="$(command -v erl${MODERL_VERSION})"
         code="io:format(\"~s\", [code:root_dir()]), halt()."
         __erl_root="$("$__erl" -boot no_dot_erlang -noshell -eval "$code")"
         ERTS_DIR="$__erl_root/erts-$ERTS_VSN"
