title: python pip error 'Cannot fetch index base URL' when in virtualenv
body: <p>it's ok to use <code>pip install -r requirements.txt</code> directly,however,when i tried to use it in <strong>virtualenv</strong>,it failed with these errors:</p>
  
  <pre><code>test@te:~$ cd flaskbb/
  test@te:~/flaskbb$ 
  test@te:~/flaskbb$ source .venv/bin/activate
  (.venv)test@te:~/flaskbb$ pip install -r requirements.txt 
  Obtaining file:///home/test/flaskbb (from -r requirements.txt (line 50))
    Running setup.py egg_info for package from file:///home/test/flaskbb
  
      no previously-included directories found matching 'flaskbb/themes/*/node_modules'
      no previously-included directories found matching 'flaskbb/themes/*/.sass-cache'
      warning: no previously-included files matching '__pycache__' found anywhere in distribution
      warning: no previously-included files matching '*.py[co]' found anywhere in distribution
  Downloading/unpacking alembic==0.9.1 (from -r requirements.txt (line 1))
    Cannot fetch index base URL http://pypi.python.org/simple/
    Could not find any downloads that satisfy the requirement alembic==0.9.1 (from -r requirements.txt (line 1))
  No distributions at all found for alembic==0.9.1 (from -r requirements.txt (line 1))
  Storing complete log in /home/test/.pip/pip.log
  (.venv)test@te:~/flaskbb$
  </code></pre>
  
  <p>And here is my system:</p>
  
  <pre><code>test@te:~$ uname -ar
  Linux te 3.2.0-101-generic #141-Ubuntu SMP Thu Mar 10 21:43:24 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux
  test@te:~$ lsb_release -a
  No LSB modules are available.
  Distributor ID: Ubuntu
  Description:    Ubuntu 12.04.5 LTS
  Release:    12.04
  Codename:   precise
  test@te:~$ uname -r
  3.2.0-101-generic
  </code></pre>
  
  <p>Thx for help!!!</p>