only test AR 5.0 on recent Ruby

.travis.yml

@@ -11,7 +11,6 @@ gemfile:
   - Gemfile_ar32
   - Gemfile_ar40threadsafe
   - Gemfile_ar42threadsafe
-  - Gemfile_ar50
 
 sudo: false
 
@@ -19,6 +18,8 @@ before_script: echo $OLD_RAKE
 
 matrix:
   include:
+    - rvm: 2.2.1
+      gemfile: Gemfile_ar50
     - rvm: 1.9.2
       gemfile: Gemfile_ar30
       env: OLD_RAKE=1