OSX: we should not deman installation of reprozip

[yarikoptic]

it is not available for anything but linux and thus now pip install niceman fails on osx

[remram44]

You can use something like:

install_requires=[
    'reprozip; sys_platform=="linux" or sys_platform=="linux2"',
    ...
]

See PEP 508

[kyleam]

@remram44 Thanks!