.travis.yml in openseadragon-0.4.0 vs .travis.yml in openseadragon-0.5.0

- old
+ new

@@ -1,6 +1,6 @@
 rvm:
-  - 2.3.1
+  - 2.4
 
 env:
   global:
     - NOKOGIRI_USE_SYSTEM_LIBRARIES=true