Do not require explicit versions of dependencies

[miracle2k]

As a library, docker-py should not require requests==2.2.1. That seems wholly unnecessary and may conflict with my own preferred version.

Worse, if I specify my own version as requests>=2.2.1, then setuptools will still raise an error if it installs my requests dependency first.

[miracle2k]

Actually, setuptools gives me error: Installed distribution requests 2.3.0 conflicts with requirement requests==2.2.1 whenever there is any other requests requirements other than ==2.2.1.

[mpetazzoni]

Thanks for reporting this. This is a known issue we need to look into. We still need a way for the library to advertise, in some way, the minimum requirements.

Closing as a duplicate of #101.