Remove workaround for ruby 2.5.0 to fix travis builds

.travis.yml

@@ -25,10 +25,3 @@ matrix:
     gemfile: gemfiles/rails_60.gemfile
 notifications:
   email: false
-
-# for 2.5.0 until 2.5.1 is released:
-#   https://github.com/travis-ci/travis-ci/issues/8978
-#   https://github.com/travis-ci/travis-ci/issues/8969#issuecomment-354135622
-before_install:
-  - gem update --system
-  - gem install bundler