title: Bundle update mini_racer is updating rails even though it is not supposed to?
body: <p>I've had the same mini_racer update problem that others have had, but the common recommendations aren't working for me.</p>
  
  <p>Getting the following errors when running <code>bundle install</code> with mini_racer 0.2.0 in my gemfile...</p>
  
  <pre><code>2 warnings generated.
  linking shared-object mini_racer_extension.bundle
  clang: warning: libstdc++ is deprecated; move to libc++ [- 
  Wdeprecated]
  ld: library not found for -lstdc++
  clang: error: linker command failed with exit code 1 (use -v to see 
  invocation)
  make: *** [mini_racer_extension.bundle] Error 1
  
  make failed, exit code 2
  
  An error occurred while installing mini_racer (0.2.0), and Bundler
  cannot continue.
  Make sure that `gem install mini_racer -v '0.2.0' --source
  'https://rubygems.org/'` succeeds before bundling.
  
  In Gemfile:
  mini_racer
  </code></pre>
  
  <p>I am trying to just update mini_racer with </p>
  
  <p><code>bundle update mini_racer</code></p>
  
  <p>but then when I run bundle install, I'm getting a new (and more alarming) error...</p>
  
  <pre><code>Bundler could not find compatible versions for gem "bundler":
  In Gemfile:
  rails (~&gt; 5.1.4) was resolved to 5.1.6, which depends on
    bundler (&gt;= 1.3.0)
  
  render_async was resolved to 2.0.0, which depends on
    bundler (~&gt; 1.8)
  
  Current Bundler version:
  bundler (2.0.1)
  This Gemfile requires a different version of Bundler.
  Perhaps you need to update Bundler by running `gem install bundler`?
  
  Could not find gem 'bundler (~&gt; 1.8)', which is required by gem 
  'rails (~&gt;
  5.1.4)', in any of the sources.
  </code></pre>
  
  <p>What should I be doing to update mini_racer without updating dependencies? (at least i think that is what is happening)</p>