Drop support of rubinius

because rake is not support rubinius
ruby/rake#150

Author: unasuke

.travis.yml

@@ -6,4 +6,8 @@ rvm:
   - 2.3.3
   - 2.4.0
   - jruby
-  - rbx-2.4.1
+  - rbx-3
+
+matrix:
+  allow_failures:
+    - rvm: rbx-3

README.md

@@ -156,9 +156,9 @@ Ryan Bates has put together an excellent RailsCast on OmniAuth:
 
 ## Supported Rubies
 
-OmniAuth Twitter is tested under 2.1.x, 2.2.x, JRuby, and Rubinius.
+OmniAuth Twitter is tested under 2.1.x, 2.2.x and JRuby.
 
-If you use its gem on ruby 1.9.x or 2.0.x, use version v1.2.1 .
+If you use its gem on ruby 1.9.x, 2.0.x, or Rubinius use version v1.2.1 .
 
 ## Contributing