Upgrade to rand 0.6

[kanru]

Update to latest release. Better performance as a side effect.

Before:

running 2 tests
test tests::bench_gen                         ... bench:         159 ns/iter (+/- 3)
test tests::bench_gen_lock_rng                ... bench:         155 ns/iter (+/- 2)

After:

running 2 tests
test tests::bench_gen                         ... bench:          74 ns/iter (+/- 2)
test tests::bench_gen_lock_rng                ... bench:          72 ns/iter (+/- 4)

Bump version to 0.3.0 because Ksuid no longer impl Rand

The ksuid cli now uses public Ksuid::generate() interface so it doesn't need to import rand crate directly.