title: bundle exec rails s failed because of unicorn
body: <p>I am using ubuntu 16.04 and rbenv for the environment.
  After successfully install the bundle, I try to start the server using <code>bundle exec rails s</code> however I got this following error</p>
  
  <pre><code>/home/mario/prayer-portal/vendor/bundle/ruby/2.1.0/gems/unicorn-5.1.0/lib/unicorn/const.rb:21:in `require_relative': cannot load such file -- /home/mario/prayer-portal/vendor/bundle/ruby/2.1.0/gems/unicorn-5.1.0/lib/unicorn/version (LoadError)
      from /home/mario/prayer-portal/vendor/bundle/ruby/2.1.0/gems/unicorn-5.1.0/lib/unicorn/const.rb:21:in `&lt;top (required)&gt;'
      from /home/mario/prayer-portal/vendor/bundle/ruby/2.1.0/gems/unicorn-5.1.0/lib/unicorn.rb:122:in `require_relative'
      from /home/mario/prayer-portal/vendor/bundle/ruby/2.1.0/gems/unicorn-5.1.0/lib/unicorn.rb:122:in `block in &lt;top (required)&gt;'
      from /home/mario/prayer-portal/vendor/bundle/ruby/2.1.0/gems/unicorn-5.1.0/lib/unicorn.rb:121:in `each'
      from /home/mario/prayer-portal/vendor/bundle/ruby/2.1.0/gems/unicorn-5.1.0/lib/unicorn.rb:121:in `&lt;top (required)&gt;'
      from /home/mario/.rbenv/versions/2.1.3/lib/ruby/gems/2.1.0/gems/bundler-1.14.3/lib/bundler/runtime.rb:91:in `require'
      from /home/mario/.rbenv/versions/2.1.3/lib/ruby/gems/2.1.0/gems/bundler-1.14.3/lib/bundler/runtime.rb:91:in `block (2 levels) in require'
      from /home/mario/.rbenv/versions/2.1.3/lib/ruby/gems/2.1.0/gems/bundler-1.14.3/lib/bundler/runtime.rb:86:in `each'
      from /home/mario/.rbenv/versions/2.1.3/lib/ruby/gems/2.1.0/gems/bundler-1.14.3/lib/bundler/runtime.rb:86:in `block in require'
      from /home/mario/.rbenv/versions/2.1.3/lib/ruby/gems/2.1.0/gems/bundler-1.14.3/lib/bundler/runtime.rb:75:in `each'
      from /home/mario/.rbenv/versions/2.1.3/lib/ruby/gems/2.1.0/gems/bundler-1.14.3/lib/bundler/runtime.rb:75:in `require'
      from /home/mario/.rbenv/versions/2.1.3/lib/ruby/gems/2.1.0/gems/bundler-1.14.3/lib/bundler.rb:107:in `require'
      from /home/mario/prayer-portal/config/application.rb:7:in `&lt;top (required)&gt;'
      from /home/mario/prayer-portal/vendor/bundle/ruby/2.1.0/gems/railties-4.1.16/lib/rails/commands/commands_tasks.rb:79:in `require'
      from /home/mario/prayer-portal/vendor/bundle/ruby/2.1.0/gems/railties-4.1.16/lib/rails/commands/commands_tasks.rb:79:in `block in server'
      from /home/mario/prayer-portal/vendor/bundle/ruby/2.1.0/gems/railties-4.1.16/lib/rails/commands/commands_tasks.rb:76:in `tap'
      from /home/mario/prayer-portal/vendor/bundle/ruby/2.1.0/gems/railties-4.1.16/lib/rails/commands/commands_tasks.rb:76:in `server'
      from /home/mario/prayer-portal/vendor/bundle/ruby/2.1.0/gems/railties-4.1.16/lib/rails/commands/commands_tasks.rb:40:in `run_command!'
      from /home/mario/prayer-portal/vendor/bundle/ruby/2.1.0/gems/railties-4.1.16/lib/rails/commands.rb:17:in `&lt;top (required)&gt;'
      from bin/rails:4:in `require'
      from bin/rails:4:in `&lt;main&gt;'
  </code></pre>