chore: Update pinned nightly so compiling works again
chore: Update pinned nightly so compiling works again

The nightlies of 2018-06-22 don't work
(https://github.com/rust-lang/rust/issues/51699,
https://twitter.com/RustStatus/status/1010240719527739392?s=19).

kbknapp/clap-rs,kbknapp/clap-rs