Update .travis.yml

* Add "language: ruby" to show the languate mode explicitly.
* Add "sudo: false" and "cache: bundler" to save system resource.
* Run default script (bundle exec rake) that includes
  both RSpec and Document status test.
* Update Rubies to latest version.
* Add ruby-head as allow_failures.
  * It's good to know new version Ruby's issue as faster before the release.
  * fast_finish is to get the Travis result as faster
    without waiting the result of the "allow_failures" items.
    See https://blog.travis-ci.com/2013-11-27-fast-finishing-builds/

.travis.yml

@@ -1,13 +1,23 @@
+language: ruby
+
+sudo: false
+
+cache: bundler
+
 before_install:
   - rvm get head
   - gem update bundler
 
 rvm:
-  - 2.2.6
-  - 2.3.3
-  - 2.4.0
+  - 2.2.7
+  - 2.3.4
+  - 2.4.1
+  - ruby-head
 
-script: bundle exec rspec **/spec/*_spec.rb
+matrix:
+  allow_failures:
+    - rvm: ruby-head
+  fast_finish: true
 
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