.travis.yml in closure_tree-6.3.0 vs .travis.yml in closure_tree-6.4.0

@@ -1,16 +1,16 @@
 cache: bundler
 sudo: false
 language: ruby
 rvm:
-  - 2.3.1
-  - 2.2.5
+  - 2.4.0
+  - 2.3.3
+  - 2.2.6
 # these haven't been passing for a while:
 #  - jruby-head
 #  - rbx
 
 gemfile:
-  - gemfiles/activerecord_4.1.gemfile
   - gemfiles/activerecord_4.2.gemfile
   - gemfiles/activerecord_5.0_foreigner.gemfile
   - gemfiles/activerecord_5.0.gemfile
   - gemfiles/activerecord_edge.gemfile