.travis.yml in tree_clusters-0.5.0 vs .travis.yml in tree_clusters-0.5.1

@@ -1,8 +1,7 @@
 sudo: false
 language: ruby
 rvm:
-  - 2.1
   - 2.2
   - 2.3
   - 2.4
   - 2.4.1
 before_install: gem install bundler -v 1.15.4