Update Travis ruby versions

Drop rbx as it’s not installing properly on Travis, bump version of
other rubies.

.travis.yml

@@ -4,28 +4,27 @@ cache: bundler
 rvm:
   - 1.9.3
   - 2.0.0
-  - 2.1.7
-  - 2.2.3
-  - 2.3.0
+  - 2.1.9
+  - 2.2.5
+  - 2.3.1
   - jruby-19mode
-  - jruby-9.0.4.0
-  - rbx-2
+  - jruby-9.0.5.0
 git:
   submodules: false
 gemfile:
   - Gemfile
 matrix:
   include:
-    - rvm: 2.3.0
+    - rvm: 2.3.1
       gemfile: gemfiles/rubyracer
-    - rvm: 2.1.7
+    - rvm: 2.1.9
       gemfile: gemfiles/rubyracer
     - rvm: jruby-19mode
       gemfile: gemfiles/rubyrhino
-    - rvm: 2.3.0
+    - rvm: 2.3.1
       gemfile: gemfiles/alaska
       env: ALASKA=1
   allow_failures:
-    - rvm: 2.3.0
+    - rvm: 2.3.1
       gemfile: gemfiles/alaska
       env: ALASKA=1