Update Travis target versions.

Author: jcoglan

.travis.yml

@@ -1,14 +1,16 @@
 sudo: false
+dist: trusty
 language: ruby
 
 rvm:
   - 1.9.3
   - 2.0.0
   - 2.1.10
-  - 2.2.9
-  - 2.3.6
-  - 2.4.3
-  - 2.5.0
+  - 2.2.10
+  - 2.3.8
+  - 2.4.6
+  - 2.5.5
+  - 2.6.3
   - jruby-19mode
   - jruby-head