bump required rust to 1.36

This isn't a fatcat rust requirement, but instead a diesel requirement,
via rust-smallvec, which in v1.0 uses the alloc crate:

  servo/rust-smallvec#73

I think the reason this came up now is that diesel-cli is an
application and doesn't have a Cargo.lock file, and the build was
updated. Using some binary mechanism to install these dependencies would
be more robust, but feels like a yak shave right now.

.gitlab-ci.yml

@@ -14,7 +14,7 @@ variables:
   PIPENV_VENV_IN_PROJECT: "true"
   CARGO_HOME: ".cargo_cache"
 
-image: "rust:1.35-stretch"
+image: "rust:1.36-stretch"
 
 unified_test:
   variables:

rust/README.md

@@ -18,7 +18,7 @@ The SQL database schema (and migrations) are under `./migrations/`.
 You need the following dependencies installed locally to build, run tests, and
 do development work:
 
-- rust stable, 2018 edition, 1.34+ (eg, via "rustup", includes cargo tool)
+- rust stable, 2018 edition, 1.36+ (eg, via "rustup", includes cargo tool)
 - diesel (`cargo install diesel_cli`)
 - postgres (compatible with 9.6+; we run 11.x in production)
 - postgres libs (debian/ubuntu: `libsqlite3-dev libpq-dev`)