title: Thin web server: `start_tcp_server': no acceptor (RuntimeError) after git branch checkout
body: <p>A Rails 3.2.0 app, working fine with Thin web server, both locally and on Heroku cedar stack.</p>
  
  <p>After:</p>
  
  <pre><code>$ git branch work
  $ git checkout work
  $ rails server
  </code></pre>
  
  <p>I get:</p>
  
  <pre><code>=&gt; Booting Thin
  =&gt; Rails 3.2.0 application starting in development on http://0.0.0.0:3000
  =&gt; Call with -d to detach
  =&gt; Ctrl-C to shutdown server
  &gt;&gt; Thin web server (v1.3.1 codename Triple Espresso)
  &gt;&gt; Maximum connections set to 1024
  &gt;&gt; Listening on 0.0.0.0:3000, CTRL+C to stop
  Exiting
  /Users/peter/.rvm/gems/ruby-1.9.3-p125/gems/eventmachine-0.12.10/lib/eventmachine.rb:572:in `start_tcp_server': no acceptor (RuntimeError)
  from /Users/peter/.rvm/gems/ruby-1.9.3-p125/gems/eventmachine-0.12.10/lib/eventmachine.rb:572:in `start_server'
  from /Users/peter/.rvm/gems/ruby-1.9.3-p125/gems/thin-1.3.1/lib/thin/backends/tcp_server.rb:16:in `connect'
  from /Users/peter/.rvm/gems/ruby-1.9.3-p125/gems/thin-1.3.1/lib/thin/backends/base.rb:53:in `block in start'
  from /Users/peter/.rvm/gems/ruby-1.9.3-p125/gems/eventmachine-0.12.10/lib/eventmachine.rb:256:in `call'
  from /Users/peter/.rvm/gems/ruby-1.9.3-p125/gems/eventmachine-0.12.10/lib/eventmachine.rb:256:in `run_machine'
  from /Users/peter/.rvm/gems/ruby-1.9.3-p125/gems/eventmachine-0.12.10/lib/eventmachine.rb:256:in `run'
  from /Users/peter/.rvm/gems/ruby-1.9.3-p125/gems/thin-1.3.1/lib/thin/backends/base.rb:61:in `start'
  from /Users/peter/.rvm/gems/ruby-1.9.3-p125/gems/thin-1.3.1/lib/thin/server.rb:159:in `start'
  from /Users/peter/.rvm/gems/ruby-1.9.3-p125/gems/rack-1.4.1/lib/rack/handler/thin.rb:13:in `run'
  from /Users/peter/.rvm/gems/ruby-1.9.3-p125/gems/rack-1.4.1/lib/rack/server.rb:265:in `start'
  from /Users/peter/.rvm/gems/ruby-1.9.3-p125/gems/railties-3.2.0/lib/rails/commands/server.rb:70:in `start'
  from /Users/peter/.rvm/gems/ruby-1.9.3-p125/gems/railties-3.2.0/lib/rails/commands.rb:55:in `block in &lt;top (required)&gt;'
  from /Users/peter/.rvm/gems/ruby-1.9.3-p125/gems/railties-3.2.0/lib/rails/commands.rb:50:in `tap'
  from /Users/peter/.rvm/gems/ruby-1.9.3-p125/gems/railties-3.2.0/lib/rails/commands.rb:50:in `&lt;top (required)&gt;'
  from script/rails:6:in `require'
  from script/rails:6:in `&lt;main&gt;'
  </code></pre>
  
  <p>Also, when I do:</p>
  
  <pre><code>sudo bundle exec rails server thin -p 3000
  </code></pre>
  
  <p>I get:</p>
  
  <pre><code>/Users/peter/.rvm/rubies/ruby-1.9.3-p125/lib/ruby/site_ruby/1.9.1/rubygems/dependency.rb:247:in `to_specs': Could not find bundler (&gt;= 0) amongst [bigdecimal-1.1.0, io-console-0.3, json-1.5.4, minitest-2.5.1, rake-0.9.2.2, rdoc-3.9.4] (Gem::LoadError)
  from /Users/peter/.rvm/rubies/ruby-1.9.3-p125/lib/ruby/site_ruby/1.9.1/rubygems/dependency.rb:256:in `to_spec'
  from /Users/peter/.rvm/rubies/ruby-1.9.3-p125/lib/ruby/site_ruby/1.9.1/rubygems.rb:1210:in `gem'
  from /Users/peter/.rvm/gems/ruby-1.9.3-p125/bin/bundle:18:in `&lt;main&gt;'
  </code></pre>
  
  <p>I have bundler 1.0.22 installed. Updated and installed it. Nothing seems to work. Any ideas?</p>