add new JavaConversions and MutableDataStructure WartTraversers

I wrote a short tutorial about wartremover [here](http://tech.kinja.com/wartremover-a-flexible-scala-code-linting-tool-1540761622) 
As part of that exercise I created two new checkers that you might find useful:
- `JavaConversions`: disabling `scala.collection.JavaConversions`
- `MutableDataStructure`: disabling `scala.collection.mutable._`

(Please note, I did not add `JavaConversions` and `MutableDataStructure` to `org.brianmckenna.wartremover.Unsafe` since these might be too opinionated to be part of the default list)