shells/ion: Unbreak build with Rust 1.40.0 (D22843)

error[E0503]: cannot use self.key_bindings because it was mutably borrowed

--> /wrkdirs/usr/ports/shells/ion/work/ion-a8872014dbce730ccd00aaa722397dc394a52bf4-a8872014dbce730ccd00aaa722397dc394a52bf4/cargo-crates/liner-0.4.4/src/context.rs:98:17
 |

96 | let ed = try!(Editor::new_with_init_buffer(stdout, prompt, self, buffer));

|                                                                        ---- borrow of `*self` occurs here

97 | match self.key_bindings {
98 | KeyBindings::Emacs => Self::handle_keys(keymap::Emacs::new(ed), handler),

|                 ^^^^^^^^^^^^^^^^^^ use of borrowed `*self`

99 | KeyBindings::Vi => Self::handle_keys(keymap::Vi::new(ed), handler),

|                                                                      -- borrow later used here

error: aborting due to previous error