Skip some tests when no DISPLAY
Skip some tests when no DISPLAY

If no display available, like when running on travis-ci, then skip the
plotting tests.

Signed-off-by: Jonas Kalderstam <35a2c6fae61f8077aab61faa4019722abf05093c@kalderstam.se>

CamDavidsonPilon/lifelines,wavelets/lifelines,jstoxrocky/lifelines,nerdless/lifelines