title: Ruby on Rails : Rake db:create -> Could not find gem
body: <p>I begin with ruby on rails and I'm trying to create my first blog. So i ran:</p>
  
  <pre><code>rails new blog -database=mysql
  </code></pre>
  
  <p>There was an error:</p>
  
  <pre><code>Gem::InstallError: The 'json' native gem requires installed build tools.
  </code></pre>
  
  <p>But since i saw a json directory in my blog's directory i didn't care.
  So i run the command to create the database:</p>
  
  <pre><code>rake db:create --trace 
  </code></pre>
  
  <p>And i had another error:</p>
  
  <pre><code>Could not find gem 'rails (= 3.1.3) x86-mingw32' in any of the gem sources listed in your Gemfile.
  Run `bundle install` to install missing gems.
  </code></pre>
  
  <p>Does anybody knows what is the problem?</p>