setup.py: conditioned install_requires

The previous commits all contain the wrong workaround. The problem is not that
MONK imports a C-extension, but that it requires to build one on install.
Therefore the other workaround doesn't work. The current workaround checks if
it is built on RTD and if it is not the pylibssh2 is added.

Signed-off-by: Erik Bernoth <erik.bernoth@gmail.com>