Make sure that compat-Cargo.lock is fresh

Previously, compat-Cargo.lock could have been updated during CI, if
Cargo.toml is changed. This can lead to spurious build failures when
testing MSRV, because not all dependencies are effectively locked.

.travis.yml

@@ -16,7 +16,7 @@ matrix:
       os: linux
       #if: everything!
       before_script: cp ci/compat-Cargo.lock ./Cargo.lock
-      script: cargo build
+      script: cargo build --locked
 
     - rust: stable
       os: linux