Update rails 5.1 and Ruby 2.3.4

Rails 4.2.8 now supports ruby 2.4

Author: lgebhardt

.travis.yml

@@ -1,19 +1,20 @@
 language: ruby
 sudo: false
 env:
-  - "RAILS_VERSION=4.2.7"
+  - "RAILS_VERSION=4.2.8"
   - "RAILS_VERSION=5.0.2"
+  - "RAILS_VERSION=5.1.0"
   - "RAILS_VERSION=master"
 rvm:
   - 2.1.10
-  - 2.2.6
-  - 2.3.3
+  - 2.2.7
+  - 2.3.4
   - 2.4.1
 matrix:
   exclude:
     - rvm: 2.1.10
       env: "RAILS_VERSION=5.0.2"
-    - rvm: 2.4.1
-      env: "RAILS_VERSION=4.2.7"
+    - rvm: 2.1.10
+      env: "RAILS_VERSION=5.1.0"
   allow_failures:
     - env: "RAILS_VERSION=master"