Drop testing against Ruby 2.1. Add Ruby 2.5. Add Rails 5.2

Author: wingrunr21

.travis.yml

@@ -1,14 +1,15 @@
 language: ruby
 rvm:
-  - 2.1
-  - 2.2
-  - 2.3.3
-  - 2.4.0
+  - 2.2.10
+  - 2.3.7
+  - 2.4.4
+  - 2.5.1
 env:
   - "RAILS_BRANCH=4-1-stable"
   - "RAILS_BRANCH=4-2-stable"
   - "RAILS_BRANCH=5-0-stable"
   - "RAILS_BRANCH=5-1-stable"
+  - "RAILS_BRANCH=5-2-stable"
   - "RAILS_BRANCH=master"
 branches:
   only: