sccache 0.2.6

Sccache is a ccache-like tool. It is used as a compiler wrapper and avoids compilation when possible, storing a cache in a remote storage using the S3 API.
sccache now includes experimental support for caching Rust compilation. This includes many caveats, and is primarily focused on caching rustc invocations as produced by cargo. A (possibly-incomplete) list follows:
* `--emit` is required.
* `--crate-name` is required.
* Only `link` and `dep-info` are supported as `--emit` values, and `link` must be present.
* `--out-dir` is required.
* `-o file` is not supported.
* Compilation from stdin is not supported, a source file must be provided.
* Values from `env!` will not be tracked in caching.
* Procedural macros that read files from the filesystem may not be cached properly
* Target specs aren't hashed (e.g. custom target specs)

If you are using Rust 1.18 or later, you can ask cargo to wrap all compilation with sccache by setting `RUSTC_WRAPPER=sccache` in your build environment.