Postgresql 12 support

[jmealo]

Howdy! I hope you are well. I tried to build jsoncdc against PostgreSQL 12 and I got this:

error: failed to run custom build command for `rpgffi v0.3.3`

Caused by:
  process didn't exit successfully: `/tmp/tmp5l2b9n7_/jsoncdc-0.1.0/target/release/build/rpgffi-35aef4f00a977405/build-script-build` (exit code: 101)
--- stderr
thread 'main' panicked at 'Destination file src/pg12.rs does not exist', /root/.cargo/registry/src/github.com-1ecc6299db9ec823/rpgffi-0.3.3/build.rs:42:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[solidsnack]

You might want to look at the new maintainers' site: https://github.com/instructure/jsoncdc

[jmealo]

@solidsnack, thanks: instructure/jsoncdc#8 (comment)

[frafra]

Please mark this repository as unmaintained and link to the new repository if possible, as it is not trivial to understand that this project is not maintained, thank you!