Merge pull request ruby-grape#5 from ruby-grape/travis

Update .travis.yml

Author: kzaitsev

.travis.yml

@@ -1,17 +1,21 @@
-language: ruby
+before_install:
+  - gem update --system
+  - gem install bundler
 
-cache: bundler
+rvm:
+  - 2.4.0
+  - 2.3.3
+  - 2.2.6
 
 matrix:
+  fast_finish: true
+
   include:
-    - rvm: 2.3.0
-    - rvm: 2.2
-    - rvm: 2.1
     - rvm: ruby-head
     - rvm: jruby-head
-    - rvm: jruby-19mode
     - rvm: rbx-2
+
   allow_failures:
     - rvm: ruby-head
     - rvm: jruby-head
-before_install: gem install bundler -v 1.12.1
+    - rvm: rbx-2