Simplify the Travis build matrix (#87)

.travis.yml

@@ -1,19 +1,12 @@
 language: elixir
-elixir:
-  - 1.2.6
-  - 1.3.4
-otp_release:
-  - 18.2
-  - 19.2
+elixir: 1.3.4
+otp_release: 19.2
 matrix:
-  # We are only interested on the newest/oldest pair
-  exclude:
-    - elixir: 1.3.4
-      otp_release: 18.2
+  include:
     - elixir: 1.2.6
-      otp_release: 19.2
+      otp_release: 18.2
 sudo: required
-group: edge
+dist: trusty
 services: docker
 env:
   - THRIFT=${TRAVIS_BUILD_DIR}/ci/thrift-docker