fix lambda-http deps

[skorobkov]

When cargo get dependencies it parses all Cargo.toml which it finds.
Because of that you can't just add to your Cargo.toml
lambda = {git = "https://github.com/awslabs/aws-lambda-rust-runtime", branch = "async-await"}
cargo will stop with error:

$ cargo check
    Updating git repository `https://github.com/awslabs/aws-lambda-rust-runtime.git`
error: failed to load source for a dependency on `lambda`

Caused by:
  Unable to update https://github.com/awslabs/aws-lambda-rust-runtime.git?branch=async-await

Caused by:
  Could not find `Cargo.toml` in `/Users/user/.cargo/git/checkouts/aws-lambda-rust-runtime-7c865cce90132439/1b95ac1/lambda-runtime`

By submitting this pull request

• I confirm that my contribution is made under the terms of the Apache 2.0 license.
• I confirm that I've made a best effort attempt to update all relevant documentation.

[skorobkov]

If you put like
lambda = {path = "vendor/aws-lambda-rust-runtime/lambda"}
then everything is ok.

[davidbarsky]

Thanks!