.travis.yml in lockfile_preserver-1.0.0 vs .travis.yml in lockfile_preserver-1.0.1

@@ -1,11 +1,13 @@
 sudo: false
 cache: bundler
 language: ruby
 bundler_args: --without debugging
 rvm:
+  - 2.3.1
   - 2.3.0
   - 2.2.4
+  - 2.1.9
   - ruby-head
 matrix:
   allow_failures:
     - rvm: ruby-head
   fast_finish: true