Fix missing trailing newline in --write-mode=diff.

Fix missing trailing newline in --write-mode=diff.

This was a bug in the `diff` crate which has been fixed in 0.1.11. This PR
simply bumps `diff` to that version.

Fixes #1753.

    $ xxd main.rs
    00000000: 666e 206d 6169 6e28 2920 7b7d            fn main() {}
    $ cargo run --bin rustfmt -- main.rs --write-mode=diff
    $ echo $?
    0
    $ git checkout issue-1753
    Switched to branch 'issue-1753'
    $ cargo run --bin rustfmt -- main.rs --write-mode=diff
    Diff in /Users/utkarsh/dev/git/rustfmt/main.rs at line 1:
     fn main() {}⏎
    +⏎
    $ echo $?
    4