title: Can not install json gem with ruby 2.2.3 on Ubuntu
body: <p>I am going through a Rails tutorial and I'm not able to complete 'bundle install' due to a problem with the json gem.  When I attempt to install it directly:</p>
  
  <pre><code>me@tru2:~/rails/hello_app$ gem install json -v '1.8.3'
  Building native extensions.  This could take a while...
  ERROR:  Error installing json:
      ERROR: Failed to build gem native extension.
  
      /home/me/.rvm/rubies/ruby-2.2.3-dev/bin/ruby -r ./siteconf20150820-12793-qdkev7.rb extconf.rb
  creating Makefile
  
  make "DESTDIR=" clean
  
  make "DESTDIR="
  compiling generator.c
  linking shared-object json/ext/generator.so
  /usr/bin/ld: cannot find -lgmp
  collect2: error: ld returned 1 exit status
  make: *** [generator.so] Error 1
  
  make failed, exit code 2
  
  Gem files will remain installed in /home/me/.rvm/gems/ruby-2.2.3-dev/gems/json-1.8.3 for inspection.
  Results logged to /home/me/.rvm/gems/ruby-2.2.3-dev/extensions/x86_64-linux/2.2.0/json-1.8.3/gem_make.out
  </code></pre>
  
  <p>As mentioned the ruby version is 2.2.3.  </p>
  
  <pre><code>me@tru2:~/rails/hello_app$ ruby --version
  ruby 2.2.3p173 (2015-08-18 revision 51636) [x86_64-linux]
  </code></pre>
  
  <p>Running 'gem list' shows that json 1.8.1 is currently installed but something in the default Gemfile must require 1.8.3.  A search showed many similar questions but no answers that got me past this issue.  I have tried running with sudo and it appears to work but going back to me the issue is still there.</p>