Allow chaining for .select2('readonly', ...)

`.select2('readonly', ...)` returns a `boolean`, which is wrong/makes no sense and makes chaining impossible, see http://jsfiddle.net/u8m62/.

This was fixed for `enable` in version 3.4.5 already, see sommit https://github.com/ivaynberg/select2/commit/a6c5302e719cc159d39ac3a8b778d2b4ed9dce76 and bug #1536 (last sentence of the bug report).

This should be fixed for 'readonly' now as well!