Could not parse version constraint ~x.x: Invalid version string "~x.x"
<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>