read-only-mode new in Emacs 24.3, provide fallback for older version.

omnisharp-find-usages doesn't work in Emacsen pre 24.3 (I'm running 24.2 currently) as read-only-mode is new in that version.

I don't know how bothered you are about supporting slight older versions, but this change provides a fallback if read-only-mode isn't defined.