rbx-2 isn't supported on CI anymore, cut it

Author: kaspth

.travis.yml

@@ -11,7 +11,6 @@ rvm:
   - 2.7.4
   - 3.0.2
   - ruby-head
-  - rbx-2
   - jruby
 gemfile:
   - gemfiles/rails_5.0.gemfile
@@ -21,7 +20,6 @@ gemfile:
   - gemfiles/rails_6.1.gemfile
 matrix:
   allow_failures:
-    - rvm: rbx-2
     - rvm: jruby
     - rvm: ruby-head
   fast_finish: true