title: Mac not using correct Yarn version
body: <p>I installed Yarn I think from npm I believe a long time ago. But have since install the latest using brew. When I try to link to Yarn to brew's version it still points to the one manually installed.</p>
  
  <p>How can I get terminal to use the brew version?</p>
  
  <pre><code> ~  brew list --versions yarn
  yarn 1.13.0
   ~  brew switch yarn 1.13.0
  Cleaning /usr/local/Cellar/yarn/1.13.0
  2 links created for /usr/local/Cellar/yarn/1.13.0
   ~  which yarn
  /Users/lee/.yarn/bin/yarn
   ~  yarn --version
  0.27.5
  </code></pre>
  
  <p>PATH</p>
  
  <pre><code>/Users/lee/.yarn/bin:/usr/local/sbin:/usr/local/bin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin:/Users/lee/Sites/AWS/ec2-api-tools-1.7.5.1/bin:/Users/lee/Sites/365TicketsGlobal/365tickets/data/scripts:/Users/lee/.composer/vendor/bin
  </code></pre>
  
  <p>Running Yarn command</p>
  
  <pre><code> ~  yarn
  yarn install v0.27.5
  (node:48263) [DEP0005] DeprecationWarning: Buffer() is deprecated due to security and usability issues. Please use the Buffer.alloc(), Buffer.allocUnsafe(), or Buffer.from() methods instead.
  [1/4] Resolving packages...
  success Already up-to-date.
  Done in 0.03s.
  </code></pre>