title: pip fails to install packages from requirements.txt
body: <p>I am trying to install a python software using the requirements file.</p>
  
  <pre><code>&gt;&gt; cat requirements.txt
  Cython==0.15.1
  numpy==1.6.1
  distribute==0.6.24
  logilab-astng==0.23.1logilab-common==0.57.1
  netaddr==0.7.6
  numexpr==2.0.1
  ply==2.5
  pycallgraph==0.5.1
  pyflowtools==0.3.4.1
  pylint==0.25.1
  tables==2.3.1
  wsgiref==0.1.2
  </code></pre>
  
  <p>So I create a virtual environment</p>
  
  <pre><code>&gt;&gt; mkvirtualenv parser
  
  (parser)
  &gt;&gt; pip freeze
  distribute==0.6.24
  wsgiref==0.1.2
  
  (parser)
  &gt;&gt; pip install -r requirements.txt
  </code></pre>
  
  <p>... and then I packages downloaded but not installed with errors: <a href="http://pastie.org/4079800" rel="noreferrer">http://pastie.org/4079800</a></p>
  
  <pre><code>(parser)
  &gt;&gt; pip freeze
  distribute==0.6.24
  wsgiref==0.1.2
  </code></pre>
  
  <p>Surprisingly, if I try to manually install each package, they install just fine.
  For instance:</p>
  
  <pre><code>&gt;&gt; pip install numpy==1.6.1
  
  (parser)
  &gt;&gt; pip freeze
  distribute==0.6.24
  wsgiref==0.1.2
  numpy==1.6.1
  </code></pre>
  
  <p>I am lost. What is going on?</p>
  
  <p>PS: I am using <code>pip</code> v1.1 and <code>python</code> v2.7.2 with <code>virtualenv</code> and <code>virtualenvwrapper</code></p>