Install packages from a requirements.txt file.

        Args:
            path (str): The path to the requirements file.
            index (str): The URL for a pypi index to use.

def install_requirements(self, path, index=None):
        """Install packages from a requirements.txt file.

        Args:
            path (str): The path to the requirements file.
            index (str): The URL for a pypi index to use.
        """
        cmd = 'install -r {0}'.format(path)
        if index:

            cmd = 'install --index-url {0} -r {1}'.format(index, path)

        self.pip(cmd)