Fix for latest emacs-snapshot changes.

[thatismatt]

On the latest emacs-snapshot I get this error:

Debugger entered--Lisp error: (error "Invalid options for slot timer in spinner")
  signal(error ("Invalid options for slot timer in spinner"))
  error("Invalid options for slot %s in %s" timer spinner)
  #[385 "
:\203\n
@\202
\243\303\211\304!\305P\306\307\310\"!\303\306\ ... 8< ...
  (defstruct (spinner (:copier nil) (:conc-name spinner--) (:constructor ma ... 8< ...
  eval-buffer(#<buffer  *load*> nil "/home/matt/.emacs.d/elpa/spinner-1.7.1/spinner.el" nil t)  ; Reading at buffer position 7645
  load-with-code-conversion("/home/matt/.emacs.d/elpa/spinner-1.7.1/spinner.el" "/home/matt/.emacs.d/elpa/spinner-1.7.1/spinner.el" nil t)
  require(spinner)
  eval((require (quote spinner)) nil)
  elisp--eval-last-sexp(nil)
  eval-last-sexp(nil)
  funcall-interactively(eval-last-sexp nil)
  #<subr call-interactively>(eval-last-sexp nil nil)
  ad-Advice-call-interactively(#<subr call-interactively> eval-last-sexp nil nil)
  apply(ad-Advice-call-interactively #<subr call-interactively> (eval-last-sexp nil nil))
  call-interactively(eval-last-sexp nil nil)
  command-execute(eval-last-sexp)

From the docs for defstruct about the :read-only argument:

Currently, only one keyword is supported, :read-only'. If this has a non-nil value, that slot cannot be set viasetf'.

This change fixes this issue by adding t as the value for :read-only for the timer slot.

Thanks for the library! I hope this is useful :)

[ptrv]

This PR has the correct fix. Will this be merged?

[Malabarba]

Closed because this was fixed in another way on a different commit. Sorry for not replying to this when it came up. I'll admit it got lost in my inbox