Use nightly for -Z but use MSRV=stable for test

.travis.yml

@@ -26,11 +26,17 @@ matrix:
            ALT=i686-unknown-linux-gnu
       rust: beta
 
+    # Minimum Rust supported channel. We enable these to make sure we
+    # continue to work on the advertised minimum Rust version.
+    # However cargo only supports the latest stable so this will get
+    # 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: nightly
+      rust: 1.27.2
       script:
-        - cargo generate-lockfile -Z minimal-versions
+        - rustup toolchain install nightly
+        - cargo +nightly generate-lockfile -Z minimal-versions
+        - cargo -V
         - cargo check --tests
 
     - env: TARGET=x86_64-unknown-linux-gnu

appveyor.yml

@@ -12,6 +12,7 @@ install:
   - appveyor-retry appveyor DownloadFile https://win.rustup.rs/ -FileName rustup-init.exe
   - rustup-init.exe -y --default-host x86_64-pc-windows-msvc --default-toolchain nightly
   - set PATH=%PATH%;C:\Users\appveyor\.cargo\bin
+  - if defined MINIMAL_VERSIONS rustup toolchain install stable
   - rustup target add %OTHER_TARGET%
   - rustc -V
   - cargo -V
@@ -22,5 +23,5 @@ clone_depth: 1
 build: false
 
 test_script:
-  - if defined MINIMAL_VERSIONS cargo generate-lockfile -Z minimal-versions && cargo check --tests
+  - if defined MINIMAL_VERSIONS cargo +nightly generate-lockfile -Z minimal-versions && cargo +stable check --tests
   - if NOT defined MINIMAL_VERSIONS cargo test