bump min rust, and remove nightly check

.travis.yml

@@ -32,7 +32,7 @@ matrix:
     # increased every 6 weeks or so when the first PR to use a new feature.
     - env: TARGET=x86_64-unknown-linux-gnu
            ALT=i686-unknown-linux-gnu
-      rust: 1.27.2
+      rust: 1.28.0
       script:
         - rustup toolchain install nightly
         - cargo +nightly generate-lockfile -Z minimal-versions

tests/testsuite/check.rs

@@ -724,10 +724,6 @@ fn check_artifacts() {
 
 #[test]
 fn short_message_format() {
-    if !is_nightly() {
-        // This can be removed once 1.30 is stable (rustdoc --error-format stabilized).
-        return;
-    }
     let foo = project()
         .file("src/lib.rs", "fn foo() { let _x: bool = 'a'; }")
         .build();