Use mt_rand to pick a random word instead of popping from array

We use an in-memory sqllite database to test our database models. When using FactoryMuff I quickly go through the wordlist as we migrate up / tear down the database for each new test. Some tests run long enough that I run into null values as wordlist is empty, and my tests fail.

I found it a more novel solution to simply randomize the words, instead of removing them from the array, as this protects against hitting a hard limit of the wordlist. 

As it's purely dummy data, I can't think of a situation where someone would expect or rely on FactoryMuff to generate unique values.