Make CI fail if there are warnings

[RalfJung]

It seems like a good idea to reject a PR on CI when it introduces a warning. We probably would want to adjust the miri script to permit the caller to add extra things to RUSTFLAGS so that we can add -D warnings?

Cc #854 @lzutao

[tesuji]

I don't think we want to deny warnings on PRs build. The current warning is

warning: the feature `const_generics` is incomplete and may cause the compiler to crash
  --> /home/travis/.rustup/toolchains/master/lib/rustlib/src/rust/src/libcore/lib.rs:77:37
   |
77 | #![cfg_attr(not(bootstrap), feature(const_generics))]
   |                                     ^^^^^^^^^^^^^^

[RalfJung]

That's a warning from building libstd, not from building Miri.

[RalfJung]

This was fixed by 6ef7c08