Merge pull request #155 from activeadmin-plugins/Fivell-patch-1

remove support for  ruby 2.1

Author: Fivell

.travis.yml

@@ -1,13 +1,8 @@
 script: bundle exec rspec spec
 env:
   matrix:
-    - RAILS=4.2.6
     - RAILS=5.0.0
+    - RAILS=5.1.0
 rvm:
-  - 2.1.9
   - 2.2.5
   - 2.3.1
-matrix:
-  exclude:
-    - rvm: 2.1.9
-      env: RAILS=5.0.0 # Rails 5.0 requires Ruby 2.2.2 or newer