Set correct name on find-or-create-repl-buffer

If the current nrepl-buffer is killed and then brought back up with
`nrepl-switch-to-repl-buffer` (C-c C-z), the new buffer has the name
"*nrepl*", when it should be "*nrepl*/project-name/". This patch fixes
the problem by calling `nrepl-repl-buffer-name` instead of using the
static "*nrepl*"-string.