Travis: Workaround a rbx-3 autoload issue

- see https://github.com/bundler/bundler/issues/6163

.travis.yml

@@ -8,7 +8,6 @@ rvm:
   - 2.3.5
   - 2.4.2
   - ruby-head
-  - rbx-3
 matrix:
   include:
   - rvm: jruby-9.1.14.0
@@ -17,6 +16,9 @@ matrix:
     jdk: openjdk7
   - rvm: jruby-head
     jdk: oraclejdk8
+  - rvm: rbx-3
+    env:
+      - RUBYOPT="-rbundler/deprecate bundle install"
   allow_failures:
     - rvm: ruby-head
     - rvm: rbx-3