Reenable the nightly channel in ci

Big derives failed for a few weeks in nightly, but it should no longer be the case.

Author: tomhoule

.travis.yml

@@ -2,7 +2,7 @@ language: rust
 rust:
   - stable
   - beta
-  # - nightly
+  - nightly
 cache: cargo
 before_script:
   - if [ "$TRAVIS_RUST_VERSION" = "stable" ]; then (rustup component add rustfmt-preview clippy-preview) fi