.travis.yml in metanorma-0.2.6 vs .travis.yml in metanorma-0.2.7

@@ -2,9 +2,10 @@
 sudo: false
 language: ruby
 rvm:
   - 2.5
   - 2.4
+  - 2.3
   - ruby-head
 node_js:
   - "iojs"
   - "8"
 before_install: