title: Packages installed by `pip install -r requirements.txt` are not found
body: <p>I'm trying to run an inherited Django project. I've set up a virtualenv and tried to pass in the requirements file via <code>pip install -r requirements.txt</code>. Everything seems to work. It tells me it's working in the correct virtualenv, and packages appear to install, e.g.:</p>
  
  <pre><code>Downloading/unpacking django-mediasync==2.2.0 (from -r requirements.txt (line 22))
    Downloading django-mediasync-2.2.0.tar.gz
    Running setup.py egg_info for package django-mediasync
  </code></pre>
  
  <p>But when I try to syncdb or runserver, </p>
  
  <pre><code>Unhandled exception in thread started by &lt;bound method Command.inner_run of &lt;django.contrib.staticfiles.management.commands.runserver.Command object at 0x10f15e290&gt;&gt;
  Traceback (most recent call last):
    File "/Users/me/.virtualenvs/example/lib/python2.7/site-packages/django/core/management/commands/runserver.py", line 88, in inner_run
      self.validate(display_num_errors=True)
    File "/Users/me/.virtualenvs/example/lib/python2.7/site-packages/django/core/management/base.py", line 249, in validate
      num_errors = get_validation_errors(s, app)
    File "/Users/me/.virtualenvs/example/lib/python2.7/site-packages/django/core/management/validation.py", line 36, in get_validation_errors
      for (app_name, error) in get_app_errors().items():
    File "/Users/me/.virtualenvs/example/lib/python2.7/site-packages/django/db/models/loading.py", line 146, in get_app_errors
      self._populate()
    File "/Users/me/.virtualenvs/example/lib/python2.7/site-packages/django/db/models/loading.py", line 61, in _populate
      self.load_app(app_name, True)
    File "/Users/me/.virtualenvs/example/lib/python2.7/site-packages/django/db/models/loading.py", line 76, in load_app
      app_module = import_module(app_name)
    File "/Users/me/.virtualenvs/example/lib/python2.7/site-packages/django/utils/importlib.py", line 35, in import_module
      __import__(name)
  ImportError: No module named mediasync
  </code></pre>
  
  <p>What's weird is that I install <code>mediasync</code> (or any other necessary packages) manually (<code>pip install django-mediasync</code>), the package can now be found.</p>
  
  <p>What am I doing wrong? I don't want to have to install all of these packages manually.</p>