Downgrade rubygems version to fix CI

[mtsmfm]

ref:

• Fix broken CI #562
• Use Bundler.with_original_env so that BUNDLER_APP_CONFIG is honored #546 (comment)

I'm looking forward the way to pass entire tests with latest rubygems though, I hope this PR can be a quick fix for our broken CI.

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[grosser]

still kinda broken ... is it better than before ?

[mtsmfm]

Yeah, I think it's much better than before because this change fixes the following LoadError we met on master branch:

/home/travis/.rvm/rubies/ruby-2.2.7/lib/ruby/site_ruby/2.2.0/bundler/runtime.rb:313:in `check_for_activated_spec!': You have already activated i18n 1.0.1, but your Gemfile requires i18n 0.9.5. Prepending `bundle exec` to your command may solve this. (Gem::LoadError)

It once passed when I tried on my forked repo:

https://travis-ci.org/mtsmfm/spring/builds/388306637

So I thought it can fix but actually still fail...

[grosser]

forward progress 🎉 keep the PRs coming :)