Allow use of newer versions of Savon

I'm trying to use this gem in an application that's using Savon 2.3.0  Since the standard winrm requires a much older version of Savon, this caused me problems.   With a few simple tweaks, I was able to replace the Savon calls with calls against builder (which savon uses internally).  I also updated the gem requirements with newer versions of the underlying libraries.
Does the builder gem follow semver? This constraint seems rather wide open, however having a very brief look at builder v3.0.0 it appears to provide a compatible public API.

I'm just worried about them releasing a new incompatible builder gem that ends up breaking winrm at some future point.