.travis.yml in metanorma-vsd-0.4.1 vs .travis.yml in metanorma-vsd-0.4.2

@@ -4,9 +4,10 @@
 cache: bundler
 os:
 - linux
 - osx
 rvm:
+- 2.6
 - 2.5
 - 2.4
 - ruby-head
 before_install: 
 - nvm install 8