ci: Use -Cpanic=abort instead of -Zno-landing-pads

`no-landing-pads` was removed from rust nightly.

mozilla/grcov#427 and
rust-lang/rust#70175

Author: mklein994

.travis.yml

@@ -7,7 +7,8 @@ matrix:
       rust: nightly
 script:
   - export CARGO_INCREMENTAL=0
-  - export RUSTFLAGS="-Zprofile -Ccodegen-units=1 -Copt-level=0 -Clink-dead-code -Coverflow-checks=off -Zno-landing-pads"
+  - export RUSTFLAGS="-Zprofile -Ccodegen-units=1 -Copt-level=0 -Clink-dead-code -Coverflow-checks=off -Zpanic_abort_tests -Cpanic=abort"
+  - export RUSTDOCFLAGS="-Cpanic=abort"
   - cargo build --verbose
   - cargo test --verbose
   - |