Temporarily change minimal-versions CI to use Rust 1.56

Because of arti#257, compatibility with earlier versions of rust are
broken: we hope this is temporary, but fixing it will probably
require a new version of rusqlite.

.gitlab-ci.yml

@@ -52,7 +52,8 @@ rust-nightly:
 
 minimal-versions:
   stage: test
-  image: rust:1.53
+  # TODO: REVERT TO 1.53 once arti#257 is solved.
+  image: rust:1.56
   script:
     - rustup install nightly
     - cargo +nightly update -Z minimal-versions