Create testing env without optional dependencies

[cbrnr]

Our only required dependencies are numpy and scipy. Therefore, we should create a testing env in Travis which includes only these two packages, and not matplotlib, scikit-learn, and mne. This will likely result in some of our tests failing - maybe we should decorate specific tests if they require optional packages (e.g. @require_sklearn, @require_matplotlib, @require_mne).

[mbillingr]

I have been wondering how to skip certain tests depending on the environment. Decorators seem like a good idea.

[cbrnr]

Yeah, that's how e.g. the MNE folks do it. See https://github.com/mne-tools/mne-python/blob/master/mne/utils.py around line 700.