replaced __import__ by importlib.import_module.

Since python 2.7, importlib.import_module is the recommended way to
import modules programmatically.

From __import__'s doc:

    Import a module. Because this function is meant for use by the
Python
    interpreter and not for general use it is better to use
    importlib.import_module() to programmatically import a module.
Replaced remaning __import__(module) calls.

This commit replaces the statements __import__(module) as the previous
replaced the statements __import__(module, {}, {}, ['']).

At first I thought leaving the single-argument calls, but perhaps it's
better to be strict rather than having exceptions to the rule in this
case.
Django 1.6 form validation errors now include ValidationError exception instances instead of just strings
map travis-ci matrix to tox environments
install updated pypy from ppa
TOXENV is tox supported env
building Pillow with pypy requries dev headers