Update Travis CI config

.travis.yml

@@ -1,15 +1,32 @@
 language: elixir
-elixir: 1.6.1
-otp_release: 20.3
 sudo: false
+elixir:
+  - 1.6
+  - 1.8
+otp_release:
+  - 20.3
+  - 21.1
 notifications:
   recipients:
     - milmazz@gmail.com
 env:
   - MIX_ENV=test
-script:
-  - mix format --check-formatted
-  - mix test
 cache:
   directories:
     - deps
+    - _build
+    - ~/.mix
+    - ~/.hex
+jobs:
+  include:
+    - stage: format
+      env:
+        - MIX_ENV=dev
+      script: mix format --check-formatted
+      elixir: 1.6
+    - stage: types
+      env:
+        - MIX_ENV=dev
+      before_script: travis_wait 20 mix dialyzer --plt
+      script: mix dialyzer --halt-exit-status
+      elixir: 1.6