Fix breakage from removing WordSplitter.

[brendan-ai2]

• Caught by the AllenNLP Hub build: http://build.allennlp.org/viewLog.html?buildId=21901&buildTypeId=AllenNLPHub_Master&tab=buildLog
• Breakage was introduced in https://github.com/allenai/allennlp/pull/3361. I've jotted this down in the 1.0 release notes.

[matt-gardner]

Do you need to update the AllenNLP commit?

[brendan-ai2]

Indeed I do. Thanks! Fixed. I'll bump the version in allennlp-hub too once both of the builds are up and running.