Support proc-macro api version 6

[jplatte]

I'm getting this error with a current nightly.

Failed spawning proc-macro server for workspace `/home/jplatte/code/.../Cargo.toml`: Failed to run proc-macro server from path /home/jplatte/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/libexec/rust-analyzer-proc-macro-srv, error: Custom { kind: Other, error: "proc-macro server's api version (6) is newer than rust-analyzer's (5)" }

Are you bundling rust-analyzer like the vscode extension does? Maybe the RA binary just needs an update.