reestablish support for ruby 1.9.3?

[chris-rock]

#123 introduced the winrm 2 dependency that supports ruby 2.x only. We should decouple the transports dependencies from train, so that we are able to support older systems with ruby 1.9.3 as well, even if they are not able to use all features.

[clintoncwolfe]

At this point in Aug 2018, we support Ruby 2.3 - 2.5 only.