.travis.yml in standalone_migrations-5.0.0 vs .travis.yml in standalone_migrations-5.2.0

@@ -1,4 +1,8 @@
 language: ruby
 rvm:
   - 2.2.2
+env:
+  - AR="~> 4.2.0" NOKOGIRI_USE_SYSTEM_LIBRARIES=1
+  - AR="~> 5.0.0" NOKOGIRI_USE_SYSTEM_LIBRARIES=1
+  - AR=">= 5.1.0.rc2,< 5.2" NOKOGIRI_USE_SYSTEM_LIBRARIES=1
 script: bundle exec rake specs:travis