Update corenlp to version 3.4, XML output

CoreNLP 3.4 allows XML output in interactive mode,
so we use that instead of parsing the plain text output.
The existing corenlp_xml module does most of the hard work.

- I added an xml file to test the raw parsing in the tests folder, is that where you want non-python test files?
- I still dislike the way I get the corenlp output, I think that there is a possible race condition
- I added a download command for corenlp to .travis.yml, but not 100% sure that it actually works