title: Could not parse version constraint ~x.x: Invalid version string "~x.x"
body: <p>I downloaded composer.phar with:</p>
  
  <pre><code>curl -sS https://getcomposer.org/installer | php
  </code></pre>
  
  <p>Then I run this command:</p>
  
  <pre><code>php composer.phar require mailgun/mailgun-php:~x.x
  </code></pre>
  
  <p>But I got this error:</p>
  
  <p>[UnexpectedValueException]<br>
    Could not parse version constraint ~x.x: Invalid version string "~x.x" </p>
  
  <p>Searching the Internet, most responses were to update the latest composer.phar.  But I just downloaded it.  If it was the case, how do I update?  (This is on a Mac OS X ver 10.10.5)</p>