title: No such option: -U on Heroku when trying to install discord.py from Git
body: <p>I am trying to make a discord bot and put it on Heroku. My code only uses <code>discord.py</code>, that is the only requirement. To install the latest version of <code>discord.py</code> locally using <code>pip</code> I have to use the command:</p>
  
  <pre><code>pip3 install -U git+https://github.com/Rapptz/discord.py@rewrite#egg=discord.py[voice]
  </code></pre>
  
  <p>I have <code>pip</code> and <code>pip3</code> on my laptop so I have to use <code>pip3</code>. This installation works properly on my laptop and I can write code with it that runs perfectly. </p>
  
  <p>If I try to add this line to the <code>requirements.txt</code> however Heroku is complaining that there is no <code>-U</code> option.</p>
  
  <p>I don't know if this is because I use <code>pip3</code> and not <code>pip</code>, or because Heroku can't handle installing from a Git repo, or because I'm using wrong syntax for that (most likely) but I couldn't find any way to install this version 1.0.0a of <code>discord.py</code> on Heroku.</p>
  
  <p>(Note that <code>pip install discord</code> does not work, that would install 0.16 version, even though this is what the <code>discord.py</code> tell you to do - installing it from the repo was a solution used by SentDex in his YouTube video about making a discord bot using Python.)</p>
  
  <p>How can I install <code>discord.py@rewrite</code> on Heroku?</p>