Configure and run formatter

.formatter.exs

@@ -0,0 +1,4 @@
+# Used by "mix format"
+[
+  inputs: ["{mix,.credo,.formatter}.exs", "{config,lib,test}/**/*.{ex,exs}"]
+]

.travis.yml

@@ -1,19 +1,44 @@
 language: elixir
-elixir:
-  - 1.4
-  - 1.5
-  - 1.6
-  - 1.7
 sudo: false
-otp_release:
-  - 18.1
-  - 19.0
-  - 20.3
-  - 21.0
-script: MIX_ENV=test mix quality
+script:
+  - if [[ `elixir -v` = *"1.9"* ]] ; then mix format --check-formatted ; fi
+  - mix credo --strict
+  - mix test
 matrix:
-  exclude:
-    - elixir: 1.4
-      otp_release: 21.0
+  # Compatible versions are taken from
+  # https://hexdocs.pm/elixir/compatibility-and-deprecations.html#compatibility-between-elixir-and-erlang-otp
+  include:
     - elixir: 1.5
-      otp_release: 21.0
+      otp_release: 19.0
+    - elixir: 1.5
+      otp_release: 20.3
+
+    - elixir: 1.6
+      otp_release: 19.0
+    - elixir: 1.6
+      otp_release: 20.3
+    - elixir: 1.6
+      otp_release: 21.3
+
+    - elixir: 1.7
+      otp_release: 19.0
+    - elixir: 1.7
+      otp_release: 20.3
+    - elixir: 1.7
+      otp_release: 21.3
+    - elixir: 1.7
+      otp_release: 22.0
+
+    - elixir: 1.8
+      otp_release: 20.3
+    - elixir: 1.8
+      otp_release: 21.3
+    - elixir: 1.8
+      otp_release: 22.0
+
+    - elixir: 1.9
+      otp_release: 20.3
+    - elixir: 1.9
+      otp_release: 21.3
+    - elixir: 1.9
+      otp_release: 22.0