Update list of Rubies to build for on TravisCI

Rubinius is not supported on the default (12.04) distribution,
so we must build on Ubuntu Trusty.
Also, the Rubinius version in RVM has apparently been renamed.

Allow failing builds on *-head rubies.

.travis.yml

@@ -1,14 +1,24 @@
 language: ruby
+sudo: required
+dist: trusty
 script: rake
 rvm:
-  - 1.9.3
-  - 2.0.0
+  - "1.9"
+  - "2.0"
   - "2.1"
   - "2.2"
+  - "2.3"
+  - "2.4"
   - ruby-head
+  - jruby
   - jruby-head
   - jruby-19mode
-  - rbx
+  - rubinius-3
 
 before_install:
   - "gem install bundler"
+
+matrix:
+  allow_failures:
+    - rvm: "ruby-head"
+    - rvm: "jruby-head"