Downgrade dependencies on minimum versions build

.travis.yml

@@ -90,3 +90,18 @@ matrix:
   - rust: stable
     name: "rustfmt"
     script: ./ci/rustfmt.sh
+
+before_script:
+  # lazy_static 1.4.0 requires Rust 1.27+
+  # and cfg-if 0.1.10 requires Rust 1.31+ so downgrade it.
+  - |
+    if [[ "$TRAVIS_RUST_VERSION" == "1.26.0" ]]; then
+      cargo generate-lockfile
+      cargo update -p cfg-if --precise 0.1.9
+      cargo update -p lazy_static --precise 1.3.0
+    fi
+  - |
+    if [[ "$TRAVIS_RUST_VERSION" == "1.28.0" ]]; then
+      cargo generate-lockfile
+      cargo update -p cfg-if --precise 0.1.9
+    fi