patches Neon dependency to make library Node 12 compatible

[AndriusA]

As per neon-bindings/neon#410

[bbondy]

Realized after merging this doesn't seem to be working:

Caused by:
  failed to load source for a dependency on `neon`

Caused by:
  Unable to update https://github.com/kjvalencik/neon/?branch=node12#5d877ac0

Caused by:
  failed to clone into: /Users/bbondy/.cargo/git/db/neon-4fce05ba4b3c7706

Caused by:
  failed to authenticate when downloading repository
attempted ssh-agent authentication, but none of the usernames `git` succeeded

Caused by:
  ERROR: Repository not found.
; class=Ssh (23); code=Eof (-20)
neon ERR! cargo build failed

[seeekr]

Thanks, using this / your current Cargo.toml makes it work with Node 12!