Get rid of MALLET

[matt-gardner]

The MALLET dependency is 5 years old, and is basically abandoned code. My dependence on MALLET is holding me back from updating some other libraries that MALLET uses (like trove - MALLET requires 2.0.2, current version is 3.0.3, and they are not compatible). My use of MALLET is just in one small piece of the code, and using a different library for learning would be a good idea.