Fix testing

[jvc26]

This PR changes the following:

1. Use tox for testing to facilitate build matrix
2. Fixes Tests all failing with InvalidResponseError #21 by ensuring a bytestring, rather than a unicode string is passed to the OpenSSL.crypto.verify() method
3. Addresses Find dependencies with which tests pass #19 - we can run on (django>=1.4,<1.7), and python 2.6, 2.7
4. Addresses Drop pip usage of --use-mirrors #18 - removes the insecure --use-mirrors flag (and -q)
5. Moves to the docker architecture on Travis, which speeds up builds

This does not fix running on django >= 1.7, which requires a further change due to the changes in application loading. See #17 for a separate issue on that.