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 <jonas@kalderstam.se>
Added travis-ci config

Was tricky to make it work with both python 2 and 3 since travis has
some troubles with their virtualenv under python3, hence the manual
virtualenv stuff in the script.

This will actually build numpy, scipy and pandas from source on each
build, and this takes around 30 mins.

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