Merge branch 'ci-switch-to-elixir-1-8-1' into 'develop'

Switch the CI to Elixir 1.8.1

See merge request pleroma/pleroma!926

.gitlab-ci.yml

-image: elixir:1.7.2
+image: elixir:1.8.1
 
 services:
   - name: postgres:9.6.2