Bump rand crate to 0.7.0

[fanatid]

By update rand crate this PR fix advisory RUSTSEC-2019-0035.

Before:

$ cargo-deny check advisories
2020-12-16 20:20:41 [WARN] unable to find a config path, falling back to default config
warning[A004]: Unaligned memory access
   ┌─ /home/kirill/projects/avro-rs/Cargo.lock:59:1
   │
59 │ rand_core 0.3.1 registry+https://github.com/rust-lang/crates.io-index
   │ --------------------------------------------------------------------- unsound advisory detected
   │
   = ID: RUSTSEC-2019-0035
   = Advisory: https://rustsec.org/advisories/RUSTSEC-2019-0035
   = Affected versions of this crate violated alignment when casting byte slices to
     integer slices, resulting in undefined behavior.

     The flaw was corrected by Ralf Jung and Diggory Hardy.
   = Announcement: https://github.com/rust-random/rand/blob/master/rand_core/CHANGELOG.md#050---2019-06-06
   = Solution: Upgrade to >=0.4.2
   = rand_core v0.3.1
     ├── rand v0.4.6
     │   └── avro-rs v0.12.0
     └── rdrand v0.4.0
         └── rand v0.4.6 (*)

advisories ok

After:

 cargo-deny check advisories
2020-12-16 20:20:36 [WARN] unable to find a config path, falling back to default config
advisories ok

[poros]

Thanks a lot!