Update .travis.yml

- tell CI the language of this project
- sudo: false
  faster:
https://docs.travis-ci.com/user/workers/container-based-infrastructure/
- cache: bundler
  faster: https://docs.travis-ci.com/user/caching/
- Test more rubies
- allow 1.9.3 & ruby-head to fail

.travis.yml

@@ -1,7 +1,17 @@
+language: ruby
+sudo: false
+cache: bundler
 rvm:
-- 1.9.3
-- 2.0.0
-- 2.1.0
+  - 2.0.0
+  - 2.1.8
+  - 2.2.4
+  - 2.3.0
+  - 1.9.3
+  - ruby-head
+matrix:
+  allow_failures:
+    - rvm: 1.9.3
+    - rvm: ruby-head
 notifications:
   email:
   - brian@dockyard.com