Stop testing on elixir 1.9

.travis.yml

@@ -1,37 +1,20 @@
 language: elixir
 notifications:
   email: false
+# http://docs.travis-ci.com/user/languages/erlang/
 elixir:
-  - 1.6
-  - 1.7
-  - 1.8
   - 1.9
   - 1.10
   - 1.11
 otp_release:
-  - 19.3
   - 20.3
-  - 21.0
-  - 22.0
-  - 23.1
-matrix:
-  exclude:
-    - elixir: 1.6
-      otp_release: 21.0
-    - elixir: 1.6
-      otp_release: 22.0
-    - elixir: 1.7
-      otp_release: 19.3
-    - elixir: 1.7
-      otp_release: 20.3
-    - elixir: 1.8
-      otp_release: 19.3
-    - elixir: 1.8
-      otp_release: 20.3
-    - elixir: 1.9
-      otp_release: 19.3
-    - elixir: 1.9
-      otp_release: 20.3
+  - 21.3
+  - 22.3
+  - 23.0
+# matrix:
+#   exclude:
+#     - elixir: 1.9
+#       otp_release: 20.3
 
 env: MIX_ENV=test
 sudo: false # faster builds