Fix incorrect verbiage in the 'downgrade' error message

[samkearney]

When attempting to downgrade a package on release, the error message is helpful but looks like it presents versions in the incorrect order:

$ cargo release 0.2.0
error: tag `v0.2.0` already exists (for `mypackage`)
error: cannot downgrade mypackage from 0.2.0 to 0.3.0

[samkearney]

Apologies, I will update the tests when I get a minute