add cache, credo and postgis to ci

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

.gitlab-ci.yml

@@ -1,11 +1,18 @@
 image: elixir:latest
 
 services:
-  - postgres:latest
+  - mdillon/postgis:9.6-alpine
 
 variables:
   MIX_ENV: "test"
 
+cache:
+  key: "$CI_JOB_NAME"
+  paths:
+    - deps
+    - _build
+    - .rebar3
+
 before_script:
   - mix local.rebar --force
   - mix local.hex --force
@@ -13,4 +20,5 @@ before_script:
 
 mix:
   script:
+  - mix credo
   - mix test