title: How to fix a bundle install "nio4r error" on Rails 5.0.0
body: <p>The exact error is:</p>
  
  <pre><code>An error occurred while installing nio4r (1.2.1), and bundler cannot continue. Make sure that 'gem install nio4r -v '1.2.1'' succeeds before bundling.
  </code></pre>
  
  <p>Here is a trace:</p>
  
  <pre><code>Fetching gem metadata from https://rubygems.org/
  Fetching version metadata from https://rubygems.org/
  Fetching dependency metadata from https://rubygems.org/
  Using rake 12.0.0
  Using concurrent-ruby 1.0.5
  Using i18n 0.8.1
  Using minitest 5.10.1
  Using thread_safe 0.3.6
  Using builder 3.2.3
  Using erubis 2.7.0
  Using mini_portile2 2.1.0
  Using rack 2.0.1
  Installing nio4r 1.2.1 with native extensions
  
  Errno::EACCES: Permission denied @ rb_sysopen - /Users/lukeplourde/.rbenv/versions/2.3.1/lib/ruby/gems/2.3.0/gems/nio4r-1.2.1/.gitignore
  Using websocket-extensions 0.1.2
  Using mime-types-data 3.2016.0521
  Using arel 7.1.4
  Using byebug 9.0.6
  Using coffee-script-source 1.12.2
  Using execjs 2.7.0
  Using method_source 0.8.2
  Using thor 0.19.4
  Using debug_inspector 0.0.2
  Using ffi 1.9.17
  Using multi_json 1.12.1
  Using rb-fsevent 0.9.8
  Using puma 3.7.0
  Using bundler 1.12.5
  Using sass 3.4.23
  Using tilt 2.0.6
  Using sqlite3 1.3.13
  Using turbolinks-source 5.0.0
  Using tzinfo 1.2.2
  Using nokogiri 1.7.0.1
  Using rack-test 0.6.3
  Using sprockets 3.7.1
  An error occurred while installing nio4r (1.2.1), and Bundler cannot continue.
  Make sure that `gem install nio4r -v '1.2.1'` succeeds before bundling.
  </code></pre>