Put CI back to trusty

Xenial is now the default.

Trusty is now out of support but still not end of life.

Also omit the ruby patch versions so we don't have to keep updating 
them.

.travis.yml

@@ -1,13 +1,14 @@
 language: ruby
+dist: trusty
 cache: bundler
 rvm:
-  - 2.0.0
-  - 2.1.10
-  - 2.2.10
-  - 2.3.8
-  - 2.4.5
-  - 2.5.5
-  - 2.6.2
+  - 2.0
+  - 2.1
+  - 2.2
+  - 2.3
+  - 2.4
+  - 2.5
+  - 2.6
   - ruby-head
 matrix:
   include: