Stop caring about Ruby 2.0.0 and Rubinius.

Even 2.1.0 has fallen out of support:
  https://www.ruby-lang.org/en/news/2017/04/01/support-of-ruby-2-1-has-ended/

I don't care about Rubinius and it has been failing on Travis anyway

Updated the micro versions of others

Author: mvidner

.travis.yml

@@ -1,18 +1,16 @@
 sudo: false
 language: ruby
 rvm:
-  - "2.3.1"
-  - "2.2.5"
+  - "2.4.1"
+  - "2.3.4"
+  - "2.2.7"
   - "2.1.10"
-  - "2.0.0"
-  - jruby-9.0.5.0
+  - jruby-9.1.5.0
   - ruby-head
-  - rbx
 matrix:
   allow_failures:
-    - rvm: jruby-9.0.5.0
+    - rvm: jruby-9.1.5.0
     - rvm: ruby-head
-    - rvm: rbx
 env:
   global:
     - TESTOPTS=-v