run cfail tests only when rustc --version == $MSRV

this version is more or less fixed so new releases of Rust (stable or nightly
channel) are unlikely to break to build due to changes in diagnostic messages

ci/script.sh

@@ -37,8 +37,11 @@ main() {
     mkdir -p ci/builds
 
     if [ $T = x86_64-unknown-linux-gnu ]; then
+        if [ $TRAVIS_RUST_VERSION == 1.*.* ]; then
+            # test on a fixed version (MSRV) to avoid problems with changes in rustc diagnostics
             # compile-fail tests
             cargo test --test single --target $T
+        fi
 
         if [ $TRAVIS_RUST_VERSION = nightly ]; then
             # multi-core compile-pass tests

ui/single/task-priority-too-high.stderr

@@ -4,7 +4,7 @@ warning: unused import: `rtfm::app`
 3 | use rtfm::app;
   |     ^^^^^^^^^
   |
-  = note: `#[warn(unused_imports)]` on by default
+  = note: #[warn(unused_imports)] on by default
 
 error[E0080]: evaluation of constant value failed
  --> $DIR/task-priority-too-high.rs:5:1