Upgrade Travis test environments to ruby 2.1.10, 2.2.5 and 2.3.1

Set default local ruby version to 2.3.1

Author: Piotr Kuczynski

.ruby-version

@@ -1 +1 @@
-2.2.2
+2.3.1

.travis.yml

@@ -3,9 +3,9 @@ cache: bundler
 sudo: false
 rvm:
   - 2.0.0
-  - 2.1.8
-  - 2.2.4
-  - 2.3.0
+  - 2.1.10
+  - 2.2.5
+  - 2.3.1
 gemfile:
   - gemfiles/rails_3.gemfile
   - gemfiles/rails_4.gemfile
@@ -16,9 +16,11 @@ matrix:
   exclude:
     - rvm: 2.0.0
       gemfile: gemfiles/rails_5.gemfile
-    - rvm: 2.1.8
+    - rvm: 2.1.10
       gemfile: gemfiles/rails_5.gemfile
-    - rvm: 2.2.4
+    - rvm: 2.2.5
+      gemfile: gemfiles/rails_3.gemfile
+    - rvm: 2.3.1
       gemfile: gemfiles/rails_3.gemfile
 before_install:
   - gem install bundler