libffi-dev package is required to intstall

Otherwise sudo pip install -U -r requirements.txt fails with:

  Running setup.py install for cryptography
    c/_cffi_backend.c:13:17: fatal error: ffi.h: No such file or directory
     #include <ffi.h>
                     ^
    compilation terminated.
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/tmp/pip_build_root/cryptography/setup.py", line 338, in <module>
        **keywords_with_side_effects(sys.argv)
      File "/usr/lib/python2.7/distutils/core.py", line 111, in setup
        _setup_distribution = dist = klass(attrs)
      File "/usr/local/lib/python2.7/dist-packages/setuptools/dist.py", line 262, in __init__
        self.fetch_build_eggs(attrs['setup_requires'])
      File "/usr/local/lib/python2.7/dist-packages/setuptools/dist.py", line 287, in fetch_build_eggs
        replace_conflicting=True,
      File "/usr/local/lib/python2.7/dist-packages/pkg_resources.py", line 614, in resolve
        dist = best[req.key] = env.best_match(req, ws, installer)
      File "/usr/local/lib/python2.7/dist-packages/pkg_resources.py", line 857, in best_match
        return self.obtain(req, installer)
      File "/usr/local/lib/python2.7/dist-packages/pkg_resources.py", line 869, in obtain
        return installer(requirement)
      File "/usr/local/lib/python2.7/dist-packages/setuptools/dist.py", line 338, in fetch_build_egg
        return cmd.easy_install(req)
      File "/usr/local/lib/python2.7/dist-packages/setuptools/command/easy_install.py", line 613, in easy_install
        return self.install_item(spec, dist.location, tmpdir, deps)
      File "/usr/local/lib/python2.7/dist-packages/setuptools/command/easy_install.py", line 643, in install_item
        dists = self.install_eggs(spec, download, tmpdir)
      File "/usr/local/lib/python2.7/dist-packages/setuptools/command/easy_install.py", line 833, in install_eggs
        return self.build_and_install(setup_script, setup_base)
      File "/usr/local/lib/python2.7/dist-packages/setuptools/command/easy_install.py", line 1055, in build_and_install
        self.run_setup(setup_script, setup_base, args)
      File "/usr/local/lib/python2.7/dist-packages/setuptools/command/easy_install.py", line 1043, in run_setup
        raise DistutilsError("Setup script exited with %s" % (v.args[0],))
    distutils.errors.DistutilsError: Setup script exited with error: command 'x86_64-linux-gnu-gcc' failed with exit status 1
    Complete output from command /usr/bin/python -c "import setuptools, tokenize;__file__='/tmp/pip_build_root/cryptography/setup.py';exec(compile(getattr(tokenize, 'open', open)(__file__).read().replace('\r\n', '\n'), __file__, 'exec'))" install --record /tmp/pip-dQb4Qp-record/install-record.txt --single-version-externally-managed --compile:
    c/_cffi_backend.c:13:17: fatal error: ffi.h: No such file or directory

 #include <ffi.h>

                 ^