Default branch as main not supported on some versions of Cargo.

[olafur-palsson]

Some versions of Cargo don't support updating from a repo without the branch master.
see rust-lang/cargo#3517 for the issue
and that's how #421 introduced a bug.

This is the error I'm getting.

    Updating git repository `https://github.com/awslabs/aws-lambda-rust-runtime/`
warning: spurious network error (2 tries remaining): failed open - '/home/oli/.cargo/git/db/aws-lambda-rust-runtime-7c865cce90132439/FETCH_HEAD' is locked: Permission denied; class=Os (2); code=Locked (-14)
warning: spurious network error (1 tries remaining): failed open - '/home/oli/.cargo/git/db/aws-lambda-rust-runtime-7c865cce90132439/FETCH_HEAD' is locked: Permission denied; class=Os (2); code=Locked (-14)
error: failed to get `lambda` as a dependency of package `hello v0.1.0 (/home/oli/projects/rust-project/rust-app-2)`

Caused by:
  failed to load source for dependency `lambda`

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

Caused by:
  failed to fetch into: /home/oli/.cargo/git/db/aws-lambda-rust-runtime-7c865cce90132439

Caused by:
  failed open - '/home/oli/.cargo/git/db/aws-lambda-rust-runtime-7c865cce90132439/FETCH_HEAD' is locked: Permission denied; class=Os (2); code=Locked (-14)

[calavera]

As explained in the readme, we don't support old versions of Rust/Cargo https://github.com/awslabs/aws-lambda-rust-runtime#supported-rust-versions-msrv