Allow install on Raspian

[bradleyjames]

Previously:

Traceback (most recent call last):
  File "./setup.py", line 587, in <module>
    main()
  File "./setup.py", line 580, in main
    install_plugin()
  File "./setup.py", line 498, in install_plugin
    install_packages(SYSTEM_DEPENDENCIES)
  File "./setup.py", line 254, in install_packages
    command = DISTRIBUTION_TO_INSTALLER[detect_linux_distribution()] + " ".join(packages)
KeyError: 'Raspbian GNU'

I've installed it and it runs fine with no other changes.

[yimuniao]

It does not fully test on Raspian. I am afraid we can not merge it.