bundle install failing on Travis

[bkeepers]

The tests have been failing for the past few days when trying to run bundle install. Here's the error:

Errno::ENOENT: No such file or directory @ rb_sysopen -
/home/travis/.rvm/rubies/ruby-2.4.0/lib/ruby/site_ruby/2.4.0/bundler/templates/Executable
An error occurred while installing concurrent-ruby (1.0.5), and Bundler cannot
continue.
Make sure that `gem install concurrent-ruby -v '1.0.5'` succeeds before
bundling.
In Gemfile:
  github-pages was resolved to 167, which depends on
    jekyll-mentions was resolved to 1.2.0, which depends on
      html-pipeline was resolved to 2.7.1, which depends on
        activesupport was resolved to 4.2.9, which depends on
          i18n was resolved to 0.9.1, which depends on
            concurrent-ruby
Errno::ENOENT: No such file or directory @ rb_sysopen -
/home/travis/.rvm/rubies/ruby-2.4.0/lib/ruby/site_ruby/2.4.0/bundler/templates/Executable
An error occurred while installing minitest (5.10.3), and Bundler cannot
continue.
Make sure that `gem install minitest -v '5.10.3'` succeeds before bundling.
In Gemfile:
  github-pages was resolved to 167, which depends on
    jekyll-mentions was resolved to 1.2.0, which depends on
      html-pipeline was resolved to 2.7.1, which depends on
        activesupport was resolved to 4.2.9, which depends on
          minitest

Example build: https://travis-ci.org/probot/probot.github.io/builds/298042595

[bkeepers]

Fixed by rubygems/rubygems#2065.

https://travis-ci.org/probot/probot.github.io/builds/299125341