fix(travis): improve handling of error code if stable is tested

.travis.yml

@@ -4,7 +4,7 @@ rust:
 - nightly
 script:
   - make docs-all gen-all-cli cargo-api ARGS=test
-  - [[ $TRAVIS_RUST_VERSION = nightly ]] && cargo test
+  - if [[ $TRAVIS_RUST_VERSION = nightly ]]; then cargo test; fi
 env:
   global:
     secure: Plj5DqAQX/4+KPM+nOAZ2sCbGIsoSrHo1YggfesQnU7paR734XO/4IayWnsNO/3q6bDi4GIcn56RUZAD3xBJJBNLia2CYIickIIYORRqLWbLdbzQaxBbD670ahtzEuUSFJTRKURPwFteAnsWYgNMNzwXOVNwLS5IUBqWTcS+N0g=