installation: Redis and Nydus pre-requisites

• Amends the installation guide WRT Redis.

• Adds Redis and Nydus to pip requirements.

• NOTE Invenio now uses Redis for session management by default. Please install it or change CFG_WEBSESSION_STORAGE settings. See INSTALL file for more details.

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>