Fix ASP.NET

Plaintext, JSON and database tests should all work except for the fortunes tests.

Had to downgrade to ASP.NET MVC 4. When mono is released with [these changes](https://github.com/mono/mono/pull/1163), we can upgrade to ASP.NET MVC 5.2.2 and the fortunes tests will also work.

When I launch nginx myself, everything seems to work. But `toolset/run-tests.py --mode verify --install server --test aspnet-mono` doesn't work for some reason.