Auto merge of #1154 - gnzlbg:semverver2, r=gnzlbg

Verify that only non-technical breaking changes are applied to libc

Closes #270 .

cc @alexcrichton so this would be a solution to #270 that uses rust-semverver to check that the API of `libc` contains only non-technical breaking changes.

This is a WIP and uses a fork of `rust-semverver` for now, but I've sent PRs upstream already. This is the only idea I have for solving #270 . `rust-semverver` is not perfect, but it can deal with functions, consts, and simple structs just fine, and that's pretty much everything that libc uses.

cc @ibabushkin

Some other notes:

* we have to compile `rust-semverver` for each toolchain version, and it depends on `cargo` so we have to build ~160 dependencies. Using `cache: cargo` breaks everything.