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.
Add a note to the README about powershell

You need to put quotes around the key/value pair passed to winrm in powershell.
You do not need to (and will get the same error if you try) in cmd.
KNIFE-386: add explicit test for winrm protocol response before bootstrap
KNIFE-386: Code review feedback
Check return status to handle 401s caught downstream
Get comments in sync with code change to handle instead of ignore errors
KNIFE-386: avoid alarming output for operations that will retry
Remove obsolete windows-shell bootstrap template
Add auth-timeout option and fix exit codes
Fix broken Windows tests
KNIFE-450: set process exit code on error
Don't rely on default value of :returns
Update CHANGELOG
Update documentation
Merge pull request #78 from opscode/adamed/knife-386-testing

KNIFE-386, KNIFE-450: WinRM bootstrap readiness, knife exit code fixes
Update version for knife-windows 0.6.0 release