try fancy travis config

Author: mitchellhenke

.travis.yml

@@ -10,9 +10,14 @@ otp_release:
   - 20.0
 
 matrix:
+  include:
+    - elixir: 1.6.0
+      env: MIX_FORMAT=true
   exclude:
     - elixir: 1.6.0
       otp_release: 18.3
+    - elixir: 1.3.4
+      otp_release: 20.0
   allow_failures:
     - elixir: 1.3.4
     - elixir: 1.4
@@ -24,4 +29,4 @@ notifications:
 script:
   - mix test
   - mix credo
-  - mix format --dry-run --check-formatted
+  - if [ "$MIX_FORMAT" = "true" ]; then mix format --dry-run --check-formatted; fi