The version of rustc_version_runtime we currently depend on (0.1.4) creates a file in the crate's `src` directory as part of the build process, which means building the crate fails on read-only file systems such as that used by docs.rs in the documentation process.

As a result, docs.rs failed to build our 2.2.0 docs: https://docs.rs/crate/mongodb/2.2.0/builds/541944

This issue was been fixed in rustc_version_runtime v0.2.0; we should upgrade so our documentation builds successfully.

See details on the docs.rs build environment here: https://docs.rs/about/builds#read-only-directories