.travis.yml in metanorma-0.2.5 vs .travis.yml in metanorma-0.2.6

@@ -1,12 +1,18 @@
+dist: trusty
 sudo: false
 language: ruby
 rvm:
-  - 2.5.1
+  - 2.5
+  - 2.4
+  - ruby-head
 node_js:
   - "iojs"
   - "8"
 before_install: 
   - gem install bundler -v 1.16.1
   - npm install -g puppeteer
   - npm install
+matrix:
+  allow_failures:
+    - rvm: ruby-head