deer$ rule is too open

[alasdairhurst]

At least one exception to the /deer$/ rule which will be incorrectly pluralised: https://github.com/blakeembrey/pluralize/blob/master/pluralize.js#L494

The plural of balladeer is balladeers. This will leave it as balladeer. The bird killdeer may or may not be pluralised with s.