Deprecation of g:airline_readonly_symbol

Recent versions of airline change the name of "g:airline_readonly_symbol" to "g:airline_symbols.readonly", the old name is deprecated (see https://raw.githubusercontent.com/bling/vim-airline/master/autoload/airline/deprecation.vim for details).
Using the old name results in errors as soon as focus is shifted to the details view of a task.
    E121: Undefined variable: g:airline_readonly_symbol
    E15: Invalid expression: b:command." ".g:airline_readonly_symbol
This commit adapts the variablename to the new naming convention.