Workaround: only run mix format check with 1.8.1

Reasoning: the canonical elixir format changed from 1.7.3 to the current
version. We don't want to bother being compliant with standards that are
outdated anyway, though we still want to *run* on that old version if
it's possible.

.travis.yml

@@ -24,7 +24,7 @@ install:
   - mix do local.rebar --force, local.hex --force, deps.get
 
 script:
-  - mix format --check-formatted
+  - if [[ "$ELIXIR_VERSION" = "1.8.1" ]]; then mix format --check-formatted; fi
   - mix test
 
 env: