cargo bench --fails to build in MacOSX

[gnzlbg]

cargo bench --all fails to build in MacOSX

[gnzlbg]

Following the instructions in HACKING.md and just ./bench/run rust produces the error:

error: Package regex v1.1.7 (/Users/gnzlbg/projects/sideprojects/regex) does not have these features: re-rust

[gnzlbg]

(cd bench && ./run rust) appears to work

[BurntSushi]

Indeed, I think this is just a documentation issue. Ideally I'd like to overhaul the benchmarks some day, but it's a huge task given both the number of benchmarks and the number of regex engines that are benchmarked.

[gnzlbg]

I'll update #589 with a fix soon.