The latest Rust v1.76.0 released a few days ago is not compatible with rustc-serialize v0.3.24, which broke the CI.
rustc-serialize v0.3.25 was released with a fix and was updated to as part of #67, which resolves the issue.
But in order to make CI work until #67 is merged, and since it is preferable to use a fixed Rust version rather than the latest stable anyway, this PR downgrades Rust back to v1.75.0.
The latest Rust v1.76.0 released a few days ago is not compatible with rustc-serialize v0.3.24, which broke the CI.
rustc-serialize v0.3.25 was released with a fix and was updated to as part of #67, which resolves the issue.
But in order to make CI work until #67 is merged, and since it is preferable to use a fixed Rust version rather than the latest stable anyway, this PR downgrades Rust back to v1.75.0.