Explictly Test with Elixir 1.10

It looks like there were some changes to what is loaded in the test
environemnt between elixir 1.10 and 1.11. This is a stop-gap while I
work on 1) moving the pipeline to use nix, and 2) refactoring the tests
to work properly regardless of elixir version.

.gitlab-ci.yml

@@ -1,4 +1,4 @@
-image: elixir:latest
+image: elixir:1.10
 
 variables:
   MIX_ENV: "test"