Use "-C panic=abort" instead of "-Zno-landing-pads"

Needed because rust removed support for "-Zno-landing-pads":
rust-lang/rust@bf45975

This caused our Code Coverage build to fail:
https://travis-ci.com/github/newsboat/newsboat/jobs/326201747#L651-L656

Also adding "-Zpanic_abort_tests" to allow `panic=abort` in libtest:
rust-lang/rust#64158

.travis.yml

@@ -106,7 +106,7 @@ matrix:
 
           # Can't have these in the `env` section above, because these settings break `cargo install`
         - export CARGO_INCREMENTAL=0
-        - export RUSTFLAGS="-Zprofile -Ccodegen-units=1 -Cinline-threshold=0 -Clink-dead-code -Coverflow-checks=off -Zno-landing-pads"
+        - export RUSTFLAGS="-Zprofile -Ccodegen-units=1 -Cinline-threshold=0 -Clink-dead-code -Coverflow-checks=off -Cpanic=abort -Zpanic_abort_tests"
     - name: "i18nspector"
       addons:
         apt: