fix build error from remove_dir_all version

[sbosnick]

The release of remove_dir_all version 0.5.3 increased the implicit MSRV of that crate to 1.40.0. The policy of the author of that crate is to support only the current stable version of Rust and to not treat a change in the implicit MSRV as a breaking change (see #XAMPPRocky/remove_dir_all#21). The author of that crate recommends pinning to an earlier version to continue to support an earlier MSRV.

Increase the MSRV in this crate to 1.40.0 to allow it to compile with remove_dir_all version 0.5.3. Also pin remove_dir_all to version 0.5.3 to avoid a caret dependency that could break this crate's MSRV unexpectedly based on future changes to remove_dir_all.

Closes: #120

[Stebalien]

While I try to support older rust versions, within reason, I also don't consider version updates to be breaking changes.

If it becomes an actual problem (e.g., MSRV bumps to a release less than 6 months old), I'm happy to introduce a maximum version to fix any fallout. But without that, I'd rather not pin a dependency (I'm not going to monitor it for releases and update the pinned version).

[sbosnick]

I see that you beat me to this change with #126. I am closing this pull request as it is now just a duplicate of that one.