KNIFE-394: Fix em-winrm dependency

em-winrm had a pinned dependency on an old beta version of eventmachine,
which has been fixed in em-winrm=0.5.5. This change ensures we have the
same version we've been requiring in the past at a minimum, since I'm not
sure what that requirement was for without research. This also allows any
version 0.x, greater than 0.5.4, with a pessimistic constraint.