title: Can't update phpunit to v7
body: <p>I use macOs Mojave, MAMP (php 7.1.2) and lately I tried phpUnit, currently running v6.1.1.</p>
  
  <p>I think I need to update phpUnit to v7, since I'm using php7.1.2. </p>
  
  <p>I installed phpUnit via composer. Here some details about the current installation:</p>
  
  <p>phpunit --version
  PHPUnit 6.1.1 by Sebastian Bergmann and contributors.</p>
  
  <p>which phpunit
  /usr/local/bin/phpunit</p>
  
  <p>When I try to update, nothing happens:</p>
  
  <p>composer require --dev phpunit/phpunit ^7</p>
  
  <p>./composer.json has been updated
  Loading composer repositories with package information
  Updating dependencies (including require-dev)
  Nothing to install or update
  Generating autoload files</p>
  
  <p>--version
  PHPUnit 6.1.1 by Sebastian Bergmann and contributors.</p>
  
  <p>Still Version 6.1.1</p>
  
  <p>What am I doing wrong?</p>
  
  <p>Best regards
  Beat</p>