Bump minimum Rust version in Mk/Uses/cargo.mk

Given that at least ripgrep will soon require Rust >= 1.28.0 [1]
and given the recently discovered buffer overflow vulnerability in
the standard library of Rust < 1.29.1 [2] bump it to 1.29.1 to avoid
surprises.

[1] https://github.com/BurntSushi/ripgrep/issues/1019
[2] https://groups.google.com/forum/#!topic/rustlang-security-announcements/CmSuTm-SaU0