Add rustc version check to build script

[phansch]

This extends the build script with a few version checks, as a first step towards autopublishing.

As it is right now, the current checks will stop the compilation if the used rust version is older than the one specified in min_version.txt or if stable or beta were used to compile Clippy.

min_version.txt contains the output of rustc -vV as that's currently the only format that rustc-version-rs understands. It was the quickest way to get it working, without writing a custom rust version parser.

I committed the min_version.txt manually, but with #2717 that should only be done through travis, so I added it to the .gitignore to prevent manual commits with that file. Until #2717 is done, we will have to update the min_version.txt manually with rustc -vV > min_version.txt when we are supporting a new nightly release.

Examples

If Clippy is compiled on beta or stable it will show the following error:

error: clippy requires a nightly version of Rust.
> Installed rustc version is: 1.25.0 (2018-03-25). Minimum required rustc version: 1.27.0-nightly (2018-05-03).

If Clippy is compiled with a nightly older than specified in min_version.txt:

error: clippy does not support this version of rustc nightly.
> Use `rustup update` or your preferred method to update Rust.
> Installed rustc version is: 1.27.0-nightly (2018-05-02). Minimum required rustc version: 1.27.0-nightly (2018-05-03).

Screenshot with colors:

Closes #2716

[phansch]

Checking why this works locally but not on travis/appveyor.
Looks like I have to implement my own commit date comparison, because currently it only checks for string equality.

[phansch]

I took the date comparison from https://github.com/SergioBenitez/version_check/blob/master/src/lib.rs#L55 and it works properly now.

[oli-obk]

I like the setup with the full rustc -vV output. We should keep it.

[cesarb]

This change breaks compilation on a rustc compiled directly from git:

   Compiling clippy v0.0.198 (file:///home/cesarb/opt/rust-git/src/rust-clippy)
error: failed to run custom build command for `clippy v0.0.198 (file:///home/cesarb/opt/rust-git/src/rust-clippy)`
process didn't exit successfully: `/home/cesarb/opt/rust-git/src/rust-clippy/target/release/build/clippy-bea81a14b521375c/build-script-build` (exit code: 101)
--- stderr
thread 'main' panicked at 'current rustc version information does not contain a rustc commit date', libcore/option.rs:914:5
note: Run with `RUST_BACKTRACE=1` for a backtrace.

warning: build failed, waiting for other jobs to finish...
error: build failed

$ rustc -vV
rustc 1.27.0-dev
binary: rustc
commit-hash: unknown
commit-date: unknown
host: x86_64-unknown-linux-gnu
release: 1.27.0-dev
LLVM version: 6.0

This rustc was compiled today from git master (e6db79f), with the following config.toml:

[build]
extended = true
sanitizers = true
profiler = true

[install]
prefix = "/home/cesarb/opt/rust-git"
sysconfdir = "etc"

[cesarb]

Probably related to rust-lang/rust#43771, the commit hash and commit date aren't available by default in the "dev" channel, which seems to be the default when compiling from git.

[phansch]

Thanks for the report! I opened #2750 and will look into this now. Probably we should always allow to build with the dev channel.