Fix bundler 1.5.0 missing json build failure for Ruby 2.1.0 .

bundler 1.5.0 does not appear to detect Ruby-shipped gems properly.

    https://github.com/bundler/bundler/issues/2780
Fix build against rbx-19mode => rbx-2.1.1 .

`rbx-19mode` isn't supported anymore by Travis.

    http://about.travis-ci.org/docs/user/languages/ruby/