rjsmin (probably rcssmin, too) package installation fails on Windows

[daniel-j-h]

The rjsmin package always tries to install the C extensions and fails if it is unable to do so.

If this is solved, we should switch cssmin out for rcssmin and include rjsmin again.

[daniel-j-h]

Solution: Install the rjsmin and rcssmin package with the flag "--without-c-extensions", like this:

pip install --install-option="--without-c-extensions" -r requirements-dev.txt

[daniel-j-h]

There's also a per-package option coming to pip

see pypa/pip#790

which looks promising in order to disable the C extensions on a per-package basis.