Add azure minimal-versions checks

.azure-pipelines.yml

@@ -58,3 +58,14 @@ jobs:
       parameters:
         rustup_toolchain: nightly
     - template: admin/pipelines/coverage.yml
+
+# Check that the minimum versions specified in Cargo.toml
+# do in fact build.
+- job: Check minimal versions build
+  pool:
+    vmImage: ubuntu-16.04
+  steps:
+    - template: az-pipeline/unstable/rustup.yml@templates
+      parameters:
+        rustup_toolchain: nightly
+    - template: admin/pipelines/min-versions.yml

admin/pipelines/min-versions.yml

@@ -0,0 +1,6 @@
+steps:
+  - script: cargo -Z minimal-versions test --all-features
+    displayName: "rustls cargo test (debug; all features)"
+  - script: cargo -Z minimal-versions test --all-features
+    displayName: "rustls-mio cargo test (debug; all features)"
+    workingDirectory: rustls-mio