Update dependency on rand

[ManifoldFR]

This pull requests updates statrs' dependency on the rand random number generation crate. I re-ran the tests and benchmarks to check everything works, fixed up a couple deprecation warnings (that didn't appear when just doing cargo build).

Resolves #92

[dhardy]

I wonder if this is the correct solution, or if we should put out a compatibility bridge between 0.5 and 0.6.

Depending on rand 0.6.0 should be good enough here.

[ManifoldFR]

The command cargo fmt --all -- --check returned 1 as an exit code in the CI, because of some formatting issue.

[boxtown]

Looks good to me