Add missing require to azure_activedirectory.rb

[flojon]

Fixes #21

[klumbe]

This should be merged asap as it fixes missing dependencies.

[jameszaghini]

To any poor soul waiting for this to be merged, you can work around it by adding
require 'net/http' to config/initializers/omniauth.rb

[klumbe]

To any poor soul waiting for this to be merged, you can work around it by adding
require 'net/http' to config/initializers/omniauth.rb

Read that before, but I would recommend to use this one: OmniAuth-azure-oauth2
Works perfectly.