Update .travis.yaml to split jobs for older and newer elixir versions

.travis.yml

@@ -7,27 +7,27 @@ elixir:
   - 1.6.3
 otp_release:
   - 20.2
+env:
+  global:
+    - MIX_ENV=test
 matrix:
   include:
+    - otp_release: 20.2
+      elixir: 1.6.3
+      script:
+        # Run all tests except pending ones
+        - "mix test --exclude pending --trace"
+        # Make sure code is formatted
+        - "mix format --check-formatted"
+        # Check type safety
+        - "mix dialyzer"
     - otp_release: 19.3
       elixir: 1.5.3
-      env: CHECK_FORMAT=false
-      fast_finish: true
-env:
-  global:
-    - MIX_ENV=test
-    - CHECK_FORMAT=true
 script:
-  # Install dependencies
-  - mix deps.get
   # Run all tests except pending ones
-  - mix test --exclude pending --trace
-  # Make sure code is formatter
-  - mix format --check-formatted
-  # Check type safety
-  - [[ "${CHECK_FORMAT}" = "true" ]] && mix dialyzer
+  - "mix test --exclude pending --trace"
 after_script:
   # Submit docs coverage to Inch CI
-  - mix inch.report
+  - "mix inch.report"
   # Submit code coverage report to Coveralls
-  - mix coveralls.travis
+  - "mix coveralls.travis"