[CI] Use Xenial build and rubies that are on it

Author: sfgeorge

.travis.yml

@@ -1,10 +1,10 @@
+dist: xenial
 language: ruby
 rvm:
-  - 2.3.0
-  - 2.4.0
-  - 2.5.0
+  - 2.3.8
+  - 2.4.5
   - 2.5.3
-  - 2.6.0
+  - 2.6.3
   - jruby-19mode
   - rbx-19mode
   - ruby-head
@@ -15,6 +15,6 @@ matrix:
     - rvm: ruby-head
 notifications:
   irc: "irc.freenode.org#adhearsion"
-sudo: true
 before_install:
+  - rvm list
   - sudo apt-get install libpcap-dev -y