.travis.yml in isobib-0.1.9 vs .travis.yml in isobib-0.1.10

@@ -1,5 +1,14 @@
 sudo: false
 language: ruby
+
 rvm:
-  - 2.4.3
+  - 2.5
+  - 2.4
+  - 2.3
+  - ruby-head
+
 before_install: gem install bundler -v 1.16.1
+
+matrix:
+  allow_failures:
+    - rvm: ruby-head