TST Improved twisted installation in tox.ini for Python 3.3

Install twisted trunk because released twisted doesn't install properly,
see https://twistedmatrix.com/trac/ticket/6539.

Also, test-requirements are removed for 3.3 because mitmproxy doesn't
install in Python 3.3, and mock is in Python 3.3 standard library.

Note that trial scrapy is not doing what is expected because twisted doesn't
have trial runner for 3.3 yet, so this command will most likely call "trial"
outside tox virtualenv and will use incorrect interpreter.