Add recent Elixir and Erlang/OTP combinations to CI (#259)

Author: adrianomitre

.travis.yml

@@ -1,16 +1,24 @@
 language: elixir
-elixir:
-  - 1.8.2
+matrix:
+  include:
+    - elixir: "1.5.3"
+      otp_release: "19.3"
+    - elixir: "1.6.6"
+      otp_release: "19.3"
+    - elixir: "1.7.4"
+      otp_release: "19.3"
+    - elixir: "1.8.2"
+      otp_release: "20.3"
+    - elixir: "1.9.4"
+      otp_release: "20.3"
+    - elixir: "1.10.3"
+      otp_release: "21.3"
+    - elixir: "1.10.3"
+      otp_release: "23.0"
 sudo: false # to use faster container based build environment
 notifications:
   recipients:
     - [email protected]
-otp_release:
-  - 22.0
 script:
   - if [[ $(elixir --version) = *"1.6"* ]]; then mix format --check-formatted; fi
   - mix test
-matrix:
-  include:
-    - elixir: 1.5.1
-      otp_release: 19.3