.travis.yml in pdfmonkey-0.3.0 vs .travis.yml in pdfmonkey-0.4.0

@@ -1,11 +1,13 @@
 language: ruby
 
 before_install:
   - gem install bundler --version '~> 1.17'
 
+dist: trusty
+
 rvm:
   - 2.6
   - 2.5
   - 2.4
-  - rbx-3
+  - rbx-4
   - truffleruby