Panic when attempting to run servo headless without X

panic.log

drobertson@tars [flex-wip]
➜  ./mach run -d -- -z                                                                                                                                                ~/git/servo/flex-wip
thread '<main>' panicked at 'called `Result::unwrap()` on an `Err` value: NoBackendAvailable(XOpenDisplayFailed)', ../src/libcore/result.rs:746 stack backtrace:
1:     0x55733aed9c90 - sys::backtrace::tracing::imp::write::h495bc662e480d01f2cv
2:     0x55733aedebdf - panicking::default_handler::_$u7b$$u7b$closure$u7d$$u7d$::closure.44522
3:     0x55733aede858 - panicking::default_handler::h4e73712d1e927dfeH0z
4:     0x55733aec87ec - sys_common::unwind::begin_unwind_inner::h0e1be209f5e9e87dg2t
5:     0x55733aec95f8 - sys_common::unwind::begin_unwind_fmt::h45dc2fac9b5e0d23m1t
6:     0x55733aed8f91 - rust_begin_unwind
7:     0x55733af15cef - panicking::panic_fmt::h40f5ec0cdc3fc429FRL
8:     0x55733962c44d - result::unwrap_failed::h4413192137636357035 at ../src/libcore/macros.rs:29
9:     0x557339628af2 - result::Result<T, E>::unwrap::h12627346629237319876 at ../src/libcore/result.rs:687
10:     0x557339623ca8 - window::Window::new::heec5ad02ef104536txa at /home/drobertson/git/servo/flex-wip/ports/glutin/window.rs:113
11:     0x557339622f06 - create_window::h5c06a46d1a79568dCcb at /home/drobertson/git/servo/flex-wip/ports/glutin/lib.rs:47
12:     0x557337d222f5 - main::h9cc0aa5bb4f2f2aeqaa at /home/drobertson/git/servo/flex-wip/components/servo/main.rs:62
13:     0x55733aede4b4 - sys_common::unwind::try::try_fn::h11728613996190555864
14:     0x55733aed8f1b - __rust_try
15:     0x55733aeddf4b - rt::lang_start::h660c7a8519a9d7d5NSz
16:     0x557337e2ada9 - main
17:     0x7f668ddef70f - __libc_start_main
18:     0x557337d21dc8 - _start
19:                0x0 - <unknown>
Servo exited with return value 101

Per the comments by larsberg and mbrubeck on IRC servo now does require a display. As a result, using something like the following should work (https://github.com/servo/saltfs/blob/master/xvfb/xvfb.conf).