[mrattle]

Bump version to v0.8.0.

PR(s) covered by this crate version bump:

• Add pipelined meta multi-get/multi-set and return mg values byte-for-byte #89

Output of cargo publish --dry-run:

> cargo publish --dry-run
    Updating crates.io index
   Packaging async-memcached v0.8.0 (/Users/mark/src/github.com/Shopify/async-memcached)
    Updating crates.io index
    Packaged 35 files, 351.0KiB (62.6KiB compressed)
   Verifying async-memcached v0.8.0 (/Users/mark/src/github.com/Shopify/async-memcached)
   Compiling async-memcached v0.8.0 (/Users/mark/src/github.com/Shopify/async-memcached/target/package/async-memcached-0.8.0)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 1.21s
   Uploading async-memcached v0.8.0 (/Users/mark/src/github.com/Shopify/async-memcached)
warning: aborting upload due to dry run