Update rust stable 1.34.2

[danpaul000]

Update to rust stable 1.34.2

The docs at https://github.com/solana-labs/solana/tree/master/ci/docker-rust have rotted a little. There's no more "automation", it's all manual. Once you do that, these two lines need an update: https://github.com/solana-labs/solana/blob/master/ci/rust-version.sh#L16-L17

[garious]

Why not do this in Silver Strand?

[mvines]

@danpaul000's got higher pri stuff to do first.