Remove extra history entry for the current parameter version

[mechanical-fish]

At some point the SSM GetParameterHistory call started returning the current version in its response. (My guess is that this might have happened when AWS introduced official support for versioning, though I haven't confirmed that.) That turns the various old workarounds for the missing "current" version into bugs, like this:

$ chamber write myservice mysecret foo
$ chamber history myservice mysecret 
Event		Version		Date		User
Created		1		10-13 17:37:21	[redacted]
Created		1		10-13 17:37:21	[redacted]

$ chamber write myservice mysecret bar
$ chamber history myservice mysecret 
Event		Version		Date		User
Created		1		10-13 17:37:21	[redacted]
Updated		2		10-13 17:37:48	[redacted]
Updated		2		10-13 17:37:48	[redacted]

This PR fixes that. After the fix:

$ chamber write myservice mysecret foo
$ chamber history myservice mysecret 
Event		Version		Date		User
Created		1		10-13 17:48:15	[redacted]

$ chamber write myservice mysecret bar
$ chamber history myservice mysecret 
Event		Version		Date		User
Created		1		10-13 17:48:15	[redacted]
Updated		2		10-13 17:48:21	[redacted]