Switch to Rust 1.31 as minimum supported version

Too many dependencies switched and we can't really keep everything at an
older version.

Commented out for now until stable becomes 1.32.

.gitlab-ci.yml

@@ -77,10 +77,10 @@ stages:
                 cargo test -p $crate; \
             done
 
-test 1.28:
-  # 1.28 img
+.test 1.31:
+  # 1.31 img
   # https://hub.docker.com/_/rust/
-  image: "rust:1.28-slim"
+  image: "rust:1.31-slim"
   <<: *cargo_test
 
 test stable: