Make suggested changes to build update

@stefanv had a few late adds to #1084 that did not make the cut:

- Add `LICENSE.txt` in `_shared` for IPython and Spyder licenses.
- Style fixes in `painttool.py` and `version_requirements.py`
- Remove deprecated `--use-mirrors` for `matplotlib` install in `.travis.yml`

I also added a `travis_retry` before all of the `pip` installs since they sometimes timeout.