[Curp] Failed to compile proto in that message.proto contains proto3 optional fields.

[Phoenix500526]

When I executed the cargo build --release as described in the Contribution Guide document, it showed me a proto compile error as follows:

$ cargo build --release
   Compiling curp v0.1.0 (/home/Xline/curp)
error: failed to run custom build command for `curp v0.1.0 (/home/Xline/curp)`

Caused by:
  process didn't exit successfully: `/home/Xline/target/release/build/curp-755359d522eb8104/build-script-build` (exit status: 101)
  --- stderr
  thread 'main' panicked at 'Failed to compile proto, error is Custom { kind: Other, error: "protoc failed: message.proto: This file contains proto3 optional fields, but --experimental_allow_proto3_optional was not set.\n" }', curp/build.rs:4:29
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

I will issue a pr to solve this problem.