Fix compile warnings caused by deprecated macros.

[lkramer]

I was getting a bunch of warnings like this:

warning: use of deprecated item 'try': use the `?` operator instead
   --> bincode/src/ser/mod.rs:416:9
    |
416 |         try!(self.add_value(variant_index));
    |         ^^^

With the latest compiler cargo 1.40.0 (bc8e4c8be 2019-11-22)

This change tries to fix them.

[jdm]

Thanks!