Switch to nightly branch of rustc

Inline ASM isn't stable, see
https://doc.rust-lang.org/unstable-book/language-features/asm.html and rust-lang/rust#29722.

We use asm in `src/kernel/execve/loader`, which needs direct access to
registers and can't be done in pure rust.
We also use the `syscall` crate in several places, which also uses
inline asm.

.travis.yml

@@ -1,11 +1,9 @@
 language: rust
 rust:
-- stable
-- beta
+- nightly
 before_script:
 - rustup component add clippy rustfmt
 script:
 - cargo fmt --all -- --check
 - cargo clippy -- -D warnings
-- cargo build
 - cargo test