update travis test matrix

Author: lgebhardt

.travis.yml

@@ -1,18 +1,15 @@
 language: ruby
 sudo: false
 env:
-  - "RAILS_VERSION=4.2.6"
-  - "RAILS_VERSION=5.0.0"
+  - "RAILS_VERSION=4.2.11"
+  - "RAILS_VERSION=5.0.7.1"
+  - "RAILS_VERSION=5.1.6.1"
+  - "RAILS_VERSION=5.2.2"
   - "RAILS_VERSION=master"
 rvm:
-  - 2.1
-  - 2.2.4
-  - 2.3.0
+  - 2.3.8
+  - 2.4.5
+  - 2.5.3
 matrix:
-  exclude:
-    - rvm: 2.0
-      env: "RAILS_VERSION=5.0.0"
-    - rvm: 2.1
-      env: "RAILS_VERSION=5.0.0"
   allow_failures:
-    - env: "RAILS_VERSION=master"
+    - env: "RAILS_VERSION=master"