title: Install newer version of bundler with bundler
body: <p>If I change the version of bundler required in a Gemfile, and then type <code>bundle</code>, I get</p>
  
  <pre><code>Bundler could not find compatible versions for gem "bundler":
    In Gemfile:
      bundler (&gt;= 1.10.2) ruby
  
    Current Bundler version:
      bundler (1.9.9)
  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;= 1.10.2) ruby in any of the sources
  </code></pre>
  
  <p>Is it possible to ask bundler to install the new version of bundler, rather than typing in <code>gem install bundler</code>?</p>