deprecate airline_readonly_symbol. move into gutter by default, so it does not conflict with bufferline logic.

autoload/airline/extensions/readonly.vim

@@ -1,11 +1,15 @@
 " MIT License. Copyright (c) 2013 Bailey Ling.
 " vim: et ts=2 sts=2 sw=2
 
+let s:symbol = get(g:, 'airline#extensions#readonly#symbol',
+      \ get(g:, 'airline_readonly_symbol', exists('g:airline_powerline_fonts') ? '' : 'RO'))
+
 function! airline#extensions#readonly#get_mark()
-  return &ro ? g:airline_readonly_symbol : ''
+  return &ro ? s:symbol : ''
 endfunction
 
 function! airline#extensions#readonly#init()
-  let g:airline_section_c .= '%{airline#extensions#readonly#get_mark()}'
+  let g:airline_section_gutter = '%{airline#extensions#readonly#get_mark()}'
+        \ .g:airline_section_gutter
 endfunction