Fix wsgi_intercept to 0.5.1, since tests stop working with 0.6.

With the latest version (0.6), tests are broken. From the GitHub README at (https://github.com/cdent/python3-wsgi-intercept), it says:

> If you are using Python 2 and need support for a different HTTP client, require a version of wsgi_intercept<0.6.

Would probably be better to move to latest version and change the tests, but this is faster :)