Make sure the environment contains only strings

        (since putenv needs a string)

def check_environment(self, environment):
        """
        Make sure the environment contains only strings

        (since putenv needs a string)
        """

        non_strings = []
        for (k,v) in environment.items():
            if not isinstance(v, basestring):
                non_strings.append(k)
        if non_strings:
            raise ValueError("The following environment variables are not strings: {}".format(", ".join(non_strings)))
        else:
            return True