title: Installing from github (with extras) via a requirements.txt file
body: <p>I am trying to add the <a href="https://haystack.deepset.ai/overview/installation" rel="nofollow noreferrer">Haystack</a> library as a dependency of a python project. The installation instructions that Haystack provides are as follows</p>
  <pre><code>git clone https://github.com/deepset-ai/haystack.git
  cd haystack
  pip install -e .[all]
  </code></pre>
  <p>I am trying to translate this into a single line that I can include in a requirements.txt. My current best guess is</p>
  <pre><code>farm-haystack[all] @ git+https://github.com/deepset-ai/haystack.git
  </code></pre>
  <p>However this emits a bunch of warnings that various versions of farm-haystack don't provide the desired extras, such as</p>
  <pre><code>WARNING: farm-haystack 0.1.0.post2 does not provide the extra 'ray'
  </code></pre>
  <p>before failing with the error message</p>
  <pre><code>ERROR: Requested dill from https://files.pythonhosted.org/packages/3e/ad/31932a4e2804897e6fd2f946d53df51dd9b4aa55e152b5404395d00354d1/dill-0.3.1.tar.gz#sha256=d3ddddf2806a7bc9858b20c02dc174396795545e9d62f243b34481fd26eb3e2c (from farm-haystack[all]@ git+https://github.com/deepset-ai/haystack.git-&gt;-r /dss_data/tmp/pip-requirements-install/req3361828774079305889.txt (line 1)) has different version in metadata: '0.3.1.dev0'
  </code></pre>
  <p>What is the proper way to go about doing this?</p>