Fix error on installation with enable-zookeeper.

This change fixes error on installing jubatus with enable-zookeeper.

Requirement class automatically generates in its constructor the formula name using the name of deriving class.
In the case of jubatus, if the requirement name is ZooKeeperRequirement, then it sets @name = 'zookeeper'.
We can also set it manually in constructor, but naming with prefix 'Requirement' or 'Dependency' is maybe the homebrew way (I think so because such automatic behavior exists), so obeyed it.