Upgrade Travis test environments to ruby 2.1.8, 2.2.4 and 2.3.0

Author: Piotr Kuczynski

.travis.yml

@@ -3,8 +3,9 @@ cache: bundler
 sudo: false
 rvm:
   - 2.0.0
-  - 2.1.6
-  - 2.2.2
+  - 2.1.8
+  - 2.2.4
+  - 2.3.0
 gemfile:
   - gemfiles/rails_3.gemfile
   - gemfiles/rails_4.gemfile
@@ -13,7 +14,7 @@ gemfile:
   - gemfiles/rails_5.gemfile
 matrix:
   exclude:
-    - rvm: 2.2.2
+    - rvm: 2.2.4
       gemfile: gemfiles/rails_3.gemfile
 before_install:
   - gem install bundler