Test passed when `cargo fmt --all -- --check` fails

[ChunMinChang]

My terminal gives a warning when running the update introduced by #57 as follow, but the travis server doesn't fail.

We should make cargo fmt --all -- --check work.

cchang@cchang-37365 cubeb-coreaudio-rs % cargo fmt --all -- --check                                                                     < (git) pr-57
Diff in /Users/cchang/Work/cubeb-coreaudio-rs/src/backend/mod.rs at line 673:
                 .unwrap()
                 .elements()
                 / input_channels;
-            if prev_frames_written == 0 && buffered_input_frames > input_frames_needed  as usize {
-                let samples_to_pop = (buffered_input_frames - input_frames_needed as usize) * input_channels;
+            if prev_frames_written == 0 && buffered_input_frames > input_frames_needed as usize {
+                let samples_to_pop =
+                    (buffered_input_frames - input_frames_needed as usize) * input_channels;
                 stm.core_stream_data
                     .input_linear_buffer
                     .as_mut()
Diff in /Users/cchang/Work/cubeb-coreaudio-rs/src/backend/mod.rs at line 681:
                     .unwrap()
                     .pop(samples_to_pop);
-                stm.frames_read.fetch_sub((samples_to_pop / input_channels) as i64, Ordering::SeqCst);
+                stm.frames_read
+                    .fetch_sub((samples_to_pop / input_channels) as i64, Ordering::SeqCst);
             }

             let elements = (missing_frames