title: 'bundle exec' complains about gem not being installed, even after 'bundle install'
body: <p>I have a website using <a href="https://help.github.com/articles/using-jekyll-with-pages/" rel="noreferrer">Jekyll with Github Pages</a>.  After previously messing about with versions and RVM on another computer, on this one I opted to stick with just one version of Ruby and per-project environments using bundler.</p>
  
  <p>I have a pretty simple <code>Gemfile</code>:</p>
  
  <pre><code>[$]&gt; cat Gemfile
  source 'https://rubygems.org'
  gem 'github-pages'
  </code></pre>
  
  <p>and bundler config:</p>
  
  <pre><code>[$]&gt; cat .bundle/config
  ---
  BUNDLE_PATH: env
  BUNDLE_DISABLE_SHARED_GEMS: '1'
  </code></pre>
  
  <p>When I run any command (jekyll, gem, irb) through <code>bundle exec</code>, I get a dependency error:</p>
  
  <pre><code>[$]&gt; bundle exec jekyll
  Could not find RedCloth-4.2.9 in any of the sources
  Run `bundle install` to install missing gems.
  </code></pre>
  
  <p>However, the bundle is already installed:</p>
  
  <pre><code>[$]&gt; bundle install
  Using RedCloth 4.2.9
  Using i18n 0.6.11
  Using json 1.8.1
  [snip]
  Using github-pages 29
  Using bundler 1.7.7
  Your bundle is complete!
  It was installed into ./env
  </code></pre>
  
  <p>I'm at a bit of a loss as to how bundler can think the gems are installed when using one subcommand, but think they're missing when using another.</p>
  
  <pre><code>[$]&gt; which ruby
  /usr/local/bin/ruby
  [$]&gt; which bundler
  /usr/local/bin/bundler
  [$]&gt; ruby --version
  ruby 2.2.0p0 (2014-12-25 revision 49005) [x86_64-darwin14]
  [$]&gt; bundler --version
  Bundler version 1.7.7
  </code></pre>