Update .travis.yml with ActiveAdmin 1.4.0 release

Author: workgena

.travis.yml

@@ -12,7 +12,7 @@ env:
     - RAILS=4.2.10 AA=1.1.0
     - RAILS=5.1.0 AA=1.2.1
     - RAILS=5.2.1 AA=1.3.1
+    - RAILS=5.2.1 AA=1.4.0
 rvm:
   - 2.3
-  - 2.4
   - 2.5